Time-variability in the Interstellar Boundary Conditions of the Heliosphere: Effect of the Solar Journey on the Galactic Cosmic Ray Flux at Earth
aa r X i v : . [ a s t r o - ph . GA ] F e b SSrv manuscript No. (will be inserted by the editor)
Time-variability in the Interstellar Boundary Conditionsof the Heliosphere: Effect of the Solar Journey on theGalactic Cosmic Ray Flux at Earth
Priscilla C. Frisch · Hans-Reinhard Mueller
Received: date / Accepted: date
Abstract
During the solar journey through galactic space, variations in the physicalproperties of the surrounding interstellar medium (ISM) modify the heliosphere andmodulate the flux of galactic cosmic rays (GCR) at the surface of the Earth, with con-sequences for the terrestrial record of cosmogenic radionuclides One phenomenon thatneeds studying is the effect on cosmogenic isotope production of changing anomalouscosmic ray fluxes at Earth due to variable interstellar ionizations. The possible rangeof interstellar ram pressures and ionization levels in the low density solar environmentgenerate dramatically different possible heliosphere configurations, with a wide rangeof particle fluxes of interstellar neutrals, their secondary products, and GCRs arrivingat Earth. Simple models of the distribution and densities of ISM in the downwind di-rection give cloud transition timescales that can be directly compared with cosmogenicradionuclide geologic records. Both the interstellar data and cosmogenic radionuclidedata are consistent with two cloud transitions, within the past 10,000 years and a sec-ond one 20,000–30,000 years ago, with large and assumption-dependent uncertainties.The geomagnetic timeline derived from cosmic ray fluxes at Earth may require ad-justment to account for the disappearance of anomalous cosmic rays when the Sun isimmersed in ionized gas.
Keywords
ISM · Heliosphere · Cosmogenic radionuclides
The Sun has traversed multiple interstellar clouds during it 220 Myr journey around thegalactic center, including dense neutral clouds, low density partially ionized interstellarmatter (ISM) such as now surrounds the heliosphere, and hot very tenuous plasma.Variations in the cosmic ray fluxes at the surface of the Earth are strongly dependent
Priscilla C. FrischUniversity of Chicago, Chicago, ILE-mail: [email protected] MuellerDartmouth College, Hanover, NHE-mail: [email protected] on the geomagnetic field, the solar magnetic activity cycle, the heliosphere, and thephysical properties of the circumheliospheric interstellar material. For example, themodulation of the 1 AU cosmic ray flux by solar flare mass ejections has long beenknown (Gosling 1964).The heliosphere acts as a weather vane for the circumheliospheric ISM (CISM),responding to the ionization, magnetic pressure, and dynamic ram pressure (Holzer1989). The cosmic ray component at 1 AU varies with the properties of the heliospheremodulation region. The interpretation of the geological record of cosmogenic isotopesrelies on accurate models of the cosmic ray spectra. One factor that is not included inthe interpretation of the geological record of cosmogenic isotopes is that the cosmic rayspectrum incident on the Earth consists of two components that behave differently asthe Sun travels through space. Galactic cosmic rays dominate at high energies, > et al. Stellar winds and supernovae in star formationregions bordering the Local Bubble both sculpted the Local Bubble and contributed tothe cosmic ray flux at Earth. Hot low density plasma is widespread in the Local Bubblecavity , T ∼ K, n < .
005 cm − (Frisch et al. ∼
15 pc, a cluster In Fig. 1, the cumulative dust distributions in and beyond the Local Bubble boundariesare evaluated from the color excess E ( B − V ) and astrometric data in the Hipparcos catalog(Perryman (1997)), after first ignoring stars with variability as shown by the Hipparcos indexI >
0. The ISM close to Sco-Cen stars, and associated with the 18.5 ◦ tilt of Gould’s Belt, arewithin the range of | Z | <
50 pc. of local interstellar clouds is flowing past the Sun. Clouds with densities of ≈ . − and ≈ cm − have been identified in this flow. Despite the large difference in scalesizes between the heliosphere, with a distance to the upwind heliopause of ∼
150 AU(0.0007 pc), and the Local Bubble, the heliosphere traces the solar environment thatis set by the Local Bubble interior, in particular the interstellar radiation field andmagnetic field. The Local Bubble environment of the Sun affects the past, present, andfuture heliosphere boundary conditions.The heliosphere varies over geologically short timescales due to the velocities of theSun and wispy local interstellar clouds through space. Fig. 1 shows the vector motionof the Sun through the interior of the Local Bubble, based on the solar apex motionthrough the local quasi-inertial frame known as the local standard of rest (LSR, we usea solar velocity of 18 . ± . − , towards ℓ , b = 47 . ◦ ± . ◦ , . ◦ ± . ◦ , based onresults in Sch¨onrich et al. ∼
30 years due to therelative 26 km s − motion of the Sun and CISM. Self-consistent photoionization modelsof the surrounding ISM provide a good match to data on the ISM inside and adjacentto the heliosphere (Slavin and Frisch 2008, S08). These models predict densities andtemperatures for the CISM of n (H ◦ )=0.19 cm − , n e =0.065 cm − , n p =0.055 cm − , andT=6,300 K (from Model 26 in SF08). The fractional ionizations of H and He are 22%and 39%, respectively. Constraints on the models include the density and temperatureof interstellar He inside of the heliosphere, the local ISM towards ǫ CMa (and Sirius),pickup ion data giving cloud neutrality, and an interstellar radiation field that includesultraviolet, soft X-ray, and extreme ultraviolet emissions, including contributions froma conductive boundary on the local interstellar cloud. Regardless of whether the LocalBubble plasma contains hot plasma, the models and data indicate that the CISM iswarm, partially ionized and low density (see Model 42 in SF08).The ISM within ∼ pc has been shocked, since the gas-phase abundances arecharacteristic of the pattern expected from the destruction of refractory grains in 50–100 km s − shock fronts (Frisch et al. et al. − (or ≈ − ); however the cluster of local interstellar clouds (CLIC, within 30 pc) ispart of a general flow of ISM past the Sun so that many relative Sun-cloud velocitiesare larger than 14 km s − (Table 1). The LSR vector motion of the CLIC is –16.8km s − arriving from the upwind direction ℓ ∼ ◦ , b ∼ − ◦ (Figure 1). This direc-tion is towards the center of the Loop I (North Polar Spur) supernova remnant (at ℓ , b =320 ◦ ,5 ◦ , Heiles 1998), and makes an angle of 70 ◦ ± ◦ with the direction of thelocal interstellar magnetic field (Frisch et al. Note there are different velcity models for the Local Interstellar Cloud (LIC,Lallement et al. et al. ǫ CMa. RecentIBEX observations are consistent with the conclusion that the Sun is in the LIC cloud seentowards Sirius (see Frisch and McComas, this volume).
Fig. 1
Left: The distribution of interstellar dust within 200 pc of the Sun and 50 pc of thegalactic plane, according to the cumulative amount of ISM traced by color excess E(B-V).The solar and CLIC motions through the LSR are given, respectively, by the thick black andred arrows (in both figures). The intersection of the S1 (cyan) and S2 (gray) magnetic shellsof Loop I with the galactic plane are shown based on spherical shells from Wolleben (2007).The E ( B − V ) contour levels of 0.08, 0.11, 0.14, 0.16 mag correspond to N ( H ◦ + H ) columndensities of 4.46e20, 6.56e20, 7.87e20, and 9.18e20 cm − , for N ( H ◦ + H )/ E ( B − V )=5.8e21atoms cm − mag (Bohlin et al. (1978)). The large blue circles show the three subgroups ofthe Sco-Cen Association; the arc centered near ℓ ∼ ◦ shows the approximate nearside of theGum Nebula. Right: The locations of the nearest stars in Table 1 are plotted in the galacticplane, together with the interstellar magnetic field direction (gray arrow, from Frisch et al. ◦ of the galactic plane. The cloudsthat are plotted include the LIC and the Blue clouds (green, blue, from RL08), and the G-cloudand Apex clouds (lavender and orange, from Frisch 2003). The clouds are labeled, and LSRvelocities are shown. ties and the configuration of local ISM (Figure 1) indicate that the Sun has recentlyemerged from the hot Local Bubble plasma and entered the CLIC.Cloud velocity is a key interstellar variable affecting the heliosphere boundary con-ditions, because the interstellar ram pressure varies as the square of the Sun-cloudvelocity. The CLIC is a decelerating flow. From upwind to downwind, cloud velocitiesrelative to the Sun ( V HC ) are –28.4 km s − towards 36 Oph, 26.3 km s − in the innerheliosphere (according to M¨obius et al. − towards χ Ori (Table1). For otherwise similar clouds, these velocity differences lead to a 50% difference inthe ram pressures of the ISM on the heliosphere over timescales of less than ≈ years, the heliocentric cloud velocities and cloudlengths in Table 1 suggest variations of a factor of ∼ N (H ◦ ) < . cm − ,is patchy. The volume density of neutral gas in nearby clouds is known only for theCISM. If all local clouds have this same density, n (H ◦ )=0.2 cm − , then ∼
35% of thesightlines to stars within 10 pc are filled with warm low density ISM. The clouds have amean thickness 0 . ± . ∼ ,
000 years.Magnetic fields permeate the region of the CLIC, and create asymmetries in theheliosphere configuration. The Sun appears to be located in or near the rim of the”S1” magnetic shell associated with Loop I (e.g. Wolleben 2007; Frisch 2010). Magnet- ically aligned interstellar dust grains near the Sun create a birefringent medium withlower optical opacities for directions parallel to the interstellar magnetic field (ISMF),and therefore polarize starlight. The direction of ISMF over the nearest 40 pc hasbeen found from fits to the position angles of polarized starlight, giving a directiontowards ℓ , b = 38 ◦ , ◦ , with uncertainties of ∼ ± ◦ (Frisch et al. ∼ ◦ angle between the magnetic field and upwind LSR CLIC direction is consistent withan ISMF compressed in an expanding superbubble shell. The ISMF direction indi-cated by the Interstellar Boundary Explorer (IBEX) Ribbon arc center, ℓ , b = 33 ◦ , ◦ (Frisch and McComas 2010, this volume) is consistent with the polarization directionto within uncertainties. A more distant measure of the ISMF is provided by fits tothe Faraday rotation and dispersion measures for four pulsars, 150–300 pc away in thethird galactic quadrant, which give a ISMF direction similar to the polarization value,and indicate a field strength of ∼ . µ G (Salvati 2010).Not all nearby clouds are warm and diffuse. Towards the constellation of Leo, andwithin 12 pc, a tiny dense cold filamentary interstellar cloud with thickness < . ◦ absorption (Meyer et al. ∼ cm − , column densities 10 − cm − ,and thermal pressures P/k = nT = 10 − cm − K (Stanimirovi´c 2009). Once thesolar motion is removed from cloud velocities (Figure 1, right), the small Blue cloud isalso seen to form in a region where the LIC and G-cloud are colliding. Both the Leocloud and the Blue cloud (e.g. HD 80007) coincide with the ring of tiny dense coldclouds identified in H ◦ − , withtemperature variations of an order of magnitude, over distances < . − ,this would correspond to a look-back time of ∼ ,
000 years. The most complete dataset available for determining cloud configurations are the H ◦ and D ◦ data, whichare free of uncertainties regarding elemental abundances (Redfield and Linsky 2004a; Wood et al. ◦ or D ◦ can be transformed to distancescales characteristic of the neutral gas for some assumed volume density for H ◦ . Wewill assume that nearby clouds have the same neutral density as the LIC, n (H ◦ )=0.2cm − .Clouds near the Sun are typically identified through parsing absorption line veloc-ity data into different clouds by assuming rigid-body motion through space for eachcloud. A number of studies provide data on cloud velocities, but the most completestudy is that of Redfield and Linsky (2008, RL08). However, one of the conclusions ofthis study, that the LIC does not surround the heliosphere, may change with bettermeasurements of the velocity of interstellar He ◦ inside of the heliosphere (Frisch andMcComas, this volume). Distinct interstellar ”clouds” have been found within 5 pc ina number of different studies (e.g. see review Frisch et al. et al. (2005)and Redfield and Linsky (2004a). The configuration of these clouds, within 60 ◦ of thegalactic plane, is shown in Figure 1, right. The LSR velocity vectors for the cloudsare shown, together with those of the CLIC and Sun. For the purposes of estimatingcloud thicknesses from Ca + , the conversion factor of N ( Ca + )/ N (HI)= 1 . × − is adopted from α Aql and η UMa data.Estimates of the times it took (or will take) for the Sun to cross interstellar cloudsobserved towards several nearby stars are listed in Table 1, for the simplest assump-tion that the time is given by T = L/V HC , for L = N (H ◦ )/ n (H ◦ ), heliocentric velocity V HC , and n (H ◦ )= 0 . − . For stars near the upwind or downwind directions, thisassumption should give realistic estimates as long as the cloud is homogeneous andneutrals and ions are well mixed. Obviously irregular cloud shapes, or cloud motionsfrom unmeasured non-radial velocities, particularly for sightlines parallel to cloud sur-faces or for inhomogenous clouds, can affect these basic estimates. Also listed in Table1 are the angles between each star and the upwind directions of the vector CLIC, andvector LIC (from He ◦ ), velocities through the LSR. Whether clouds in the local ISMare filamentary or blobs affects the time inferred for the Sun to cross the cloud, partic-ularly for stars making a large angle with the upwind direction where radial velocitiesare small.The times in Table 1 are estimates only, since accurate dating of solar transitions ofcloud boundaries requires data on the volume densities that are not generally available.If the clouds are in thermal pressure equilibrium, the cloud temperature variations of afactor of two or more (Redfield and Linsky 2004b) suggest there are also variations incloud densities, which can not be evaluated without information on cloud ionization.In addition, the structure of local ISM is resolved at very low spatial resolution, sincecurrent data only sample the sky at a spacing of ∼ φ HeI . Estimates of the time when the Sun entered inthe LIC will be based on interstellar data towards four stars, two near the downwinddirection of the LIC ( χ Ori and α Aur), and two at more oblique angles to the LIC
Table 1
Crossing times for Clouds Close to Sun (1)
Star ℓ , b ,Dist,Cld V HC , V LSR N (H ◦ ) L , Cloud θ CLIC , φ
HeI T , CrossingThick. LSR, HC Time(deg,deg, pc) (km s − ) (cm − ) (pc) (deg) (years) α Cen 316, –1, 1.3, G –18.0, –18.6 17.61 0.66 20 ◦ , 130 ◦ (35,900) α CMa 227, –9, 2.7, L 19.6, 2.2 17.60 0.65 106 ◦ , 42 ◦ , α CMa 227, –9, 2.7, B 13.7, –3.7 17.40 0.41 106 ◦ , 42 ◦ , α CMi 214, 13, 3.5 24.0, 10.4 17.81 1.86 122 ◦ , 41 ◦ α CMi 214, 13, 3.5, L 20.5, 6.5 18.08 1.38 122 ◦ , 41 ◦ ◦ , ◦ (670,000)70 Oph 30, 12, 5.1, A –32.4, –15.5 17.46 0.47 58 ◦ , ◦ (530,000) α Aql 48, –9, 5.1, G –18.1, -0.3 17.91 1.3 72 ◦ , ◦ (1.1e6) α Aql 48, –9, 5.1, A –26.9, –10.7 17.47 0.48 72 ◦ , ◦ (670,000) χ Ori 188, –3, 9, L 22.3, 27.4 17.93 1.38 146 ◦ , 13 ◦ α Aur 163, 4, 13, L 21.8, 15.3 18.24 2.82 171 ◦ , 28 ◦ ◦ , 170 ◦ (39,800) (1) Column 1 gives the star number in Figure 1, and star name. The galactic coordinates,distance, and cloud to which the component is attributed, are listed in Column 2. G andA refer to the G-cloud and Apex cloud (from Frisch 2003), and L and B refer to the LICand Blue clouds (from Redfield and Linsky 2008). Column 3 gives the observed velocitiesof the interstellar absorption components in the solar (e.g., Redfield and Linsky (2008)) andLSR inertial systems Column 4 is the log of the cloud column density, from Wood et al. (2005) or Hebrard et al. (1999). Column 5 is the cloud thickness, N (H ◦ )/ n (H ◦ ) calculatedfor volume density n (H ◦ )= 0 . − . Column 6 gives the angles θ CLIC and φ HeI , which arethe angles between the star and the LSR upwind CLIC direction ( ℓ , b = 335 ◦ , − ◦ ), and theheliocentric (HC) downwind He ◦ vector ( ℓ , b = 184 ◦ , − ◦ ), respectively. Column 7 gives anominal crossing time for the cloud, calculated from the cloud thickness and V HC . The timesinside of parentheses are in the future. vector ( α CMi, and α CMa). The Sun is assumed to be in the LIC. Because of theuncertainties in the ionization, volume density, magnetic pressure, an homogeneityof these clouds, the timescales below are quite uncertain. However they do show theplausibility of an interstellar effect on the cosmogenic isotope record.The star χ Ori, at 9 pc, is viewed through the heliosphere tail ( φ HeI = 13 ◦ , Table1). The observed V HC = 22 km s − component corresponds to the LIC velocity. Forthe above premise that L = N (H ◦ )/ n (H ◦ ), the Sun would have entered the LIC ∼ ,
500 years ago. The relatively high turbulence of the LIC absorption lines towards χ Ori (e.g. Redfield and Linsky 2008), however, suggests that two clouds with similarvelocities could blend in velocity to form this component. If so, the Sun would haveentered the LIC more recently. A more distant star in the downwind direction is α Aur( φ HeI = 28 ◦ ). The higher LIC H ◦ column densities suggest, instead, that the the Sunentered the ISM 126,000 years ago. The star α CMi is much closer to the Sun, but itis offset 41 ◦ from the downwind LIC direction ( φ HeI =41 ◦ ). The LIC component givesan entry time into the LIC of 92,900 years ago.The closest downwind star is Sirius ( α CMa, 2.7 pc), with two clouds in front ofit. Sirius is 106 ◦ from the downwind direction. The Sun would have entered the LICcomponent 32,200 years ago, and the Blue cloud ∼ ,
000 years before that.An alternate LIC encounter time follows from the assumption that the LIC isfilamentary, with the filament oriented perpendicular to the LSR velocity and roughlyparallel to the magnetic field direction. When distant diffuse ISM is spatially resolved, filamentary structures are generally observed for neutral and ionized gas. Magneticpressure creates filamentary ISM towards Loop I at distances of ∼
90 pc, so the evidentrelation between Loop I and nearby ISM suggest the LIC could also be filamentary.Frisch (1994) assumed a filamentary structure for the LIC, with the direction of theLSR cloud velocity perpendicular to the filament, and found the Sun may have enteredthe CISM gas anytime within the past 2,000–8,000 years, or even more recently.For the comparisons with the geologic radio isotope record in Section 4, and sincethe detailed structure of the downwind gas is not known, we adopt several time intervalsas candidates for a solar transition between interstellar clouds. The primary downwindclouds of interest are the LIC, which is most likely presently surrounding the Sun, andthe Blue cloud (Figure 1, right). These transitions are: (1) Sometime within the past8,000 years, as indicated by the α CMa sightline together with an assumed filamentarycloud structure. (2) Sometime within 10 , − ,
000 years ago. (3) Before that theSun was in the Blue cloud, with an entry time of ∼ ,
000 years ago from the α CMa timescales. The Blue cloud should be more ionized than the LIC because it isless shielded from the primary source of local H-ionization, ǫ CMa. This suggests thatthe Sun traveled through a cloud with higher ionization levels than the LIC, beforeit entered the LIC. This interpretation is also consistent with the local ionizationgradient found by Wolff et al. (1999). (4) Any of the surfaces of these clouds may havea conductive interface. However the clumpiness of the local ISM, and the apparentdeceleration evident in the CLIC velocities, suggest that the clumps of gas that formthe CLIC have a cohesive origin and are close to each other. Transition times across0.5 pc cloud interfaces, where the relative Sun-cloud velocities may be larger by ∼
20 km s − , may be 10,000 years, or less, depending on the angle of the ISMF thatinhibits conduction. (5) No tiny dense clouds have been directly detected nearby in thedownwind direction, however the Blue cloud coincides with a ring of tiny cold denseclouds (Section 2.1). Typical sizes for these clouds, as small as 30 AU, indicate thatsuch clouds would sweep past the heliosphere over timescales of a decade. The heliosphere results from a balance between solar wind and interstellar pressure.This balance sets the size of the heliosphere and determines the particle distributionsand magnetic field configurations throughout, which in turn determine how galacticcosmic rays are being modulated during their passage through the heliosphere on theirway to Earth.The inner boundary conditions of the heliosphere consist of the solar wind, im-printed with the 22 year magnetic activity cycle of the Sun, as a “point source” inflow.The time variability of the solar wind affects the large-scale heliosphere, including theheliosheath regions immediately inside and outside of the heliopause. The distanceof the termination shock in the nose direction varies by about 10 AU with the solarcycle phase, and is closer during solar wind minima when ram pressures are lower(Zank and M¨uller 2003). The reversals of the solar magnetic polarity every ∼
11 yearsbetween successive solar minima propagate to the inner heliosheath regions, and createbands of magnetic polarity that are swept around the flanks of the heliosphere withthe subsonic solar wind (e.g., Pogorelov et al. T, MeV10 -2 -1 J , m - s - s t e r - M e V - LISMIMP-8BESSLIC (Model 1)LIC (Model 2)LB (Model 1)LB (Model 2)DC (Model 1)DC (Model 2)
Fig. 2
Left.
The density distribution for the heliosphere when immersed in a moderately densecloud, n (H ◦ ) ∼
15 cm − , with a temperature T=3,000 K, ion density of 0.2 cm − , and movingat the LIC velocity (Model 17 in M¨uller et al. Right.
Spectra of galactic cosmic rays at 1 AU for three different interstellar clouds surroundingthe heliosphere (ACRs are not included). LIC stands for the contemporary CISM; LB is theLocal Bubble interior modeled as a 1 . × K fully ionized plasma with density 0.005 cm − ;and DC is a dense cloud with density 10 cm − , T=200 K, and relative velocity of 25 km s − .Models 1 and 2 utilize different cosmic ray modulation models (figure from Florinski and Zank2006). pressure of the neutrals which participate through charge-exchange with ions outsidethe heliopause; the pressure interior of the heliopause is modified by charge exchangeas well.Configurations of the global heliosphere have been modeled for a range of surround-ing interstellar cloud properties. In one study (M¨uller et al. et al. (2006) explore a wide range ofpossible interstellar environments, including cold dense clouds, hot tenuous, completelyionized clouds like that assumed for the Local Bubble, and systems on galactic pathsthat result in fast relative Sun-ISM velocities. In particular the relative velocity hasa decisive influence on how much interstellar neutral material reaches the inner he-liosphere (filtration); the larger the relative velocity, the less filtration is occurring. Ifthe Sun encounters a completely ionized ISM, such as the Local Bubble, GCR particlefluxes at Earth are lower than currently. With neutrals absent, none of the pressurebalance modifications take place, and secondary particles like anomalous cosmic rays(ACR) do not exist. Lastly, if the heliosphere is embedded in a dense cloud, the he-liosphere is small in size, and particle fluxes at Earth rise substantially. Depending onthe interstellar ram pressure, the heliosphere can easily get small enough for the Earthorbit to be partly in the inner heliosheath region. Yeghikyan and Fahr (2006) consider the passage of very dense molecular cloudsover the heliosphere. The resulting heliospheres are very small, so that the orbit ofEarth takes it through regions of interstellar gas outside the heliopause. The expectedneutral fluxes at Earth are so high that changes for the ozone layer and other climate-related effects are to be expected. Similarly, when a supernova shock front washes overthe heliosphere (M¨uller et al. et al. (2003), where a cold dense cloud results in a moderatesize heliosphere of 23 AU, a two-fold flux of GCR, and a ten-fold flux of ACR isreported. Figure 2b illustrates this with calculated spectra for the Local Bubble caseand for a dense cloud case, comparing them to the GCR fluxes of the contemporaryheliosphere.
Spallation of cosmic-rays in the terrestrial atmosphere creates the radioisotopes Be, Cl, and C that are used to date the repository geological archive. Both the geo-magnetic and heliosphere magnetic fields modulate the incident cosmic-ray flux (e.g.McCracken 2004, and the Beer and McCracken articles in this volume), including theACR component. The well known anticorrelation between cosmic ray fluxes at 1 AUand the solar magnetic activity cycle (see articles by Leske and Mewaldt in this vol-ume) suggests that ISM-driven variations in the heliosphere may also be significant.Earlier interstellar explanations for the peaks in the concentration of Be in the icecore record include the reduced modulation of cosmic rays by a heliosphere that hasbeen compressed by a passing supernova shock (Sonett et al. Be and Cl ice core data, and the C tree-ring data. Recon-structing the geomagnetic field from cosmogenic isotope data requires an assumptionabout the flux and spectra of cosmic rays, and the production and dispersal of radioiso-topes. Muscheler et al. (2005, M05) compared the paleomagnetic field determined bythese different techniques, and found several discrepancies that are not explained. Giventhe mystery of these anomalies, we postulate that temporal variations in the propertiesof the ISM shaping the heliosphere led to variations in the total cosmic-ray spectrumat the Earth. These variations have been overlooked as a factor in the geomagnetictime-line developed from cosmogenic isotopes, but need to taken into consideration.The anomalies in the reconstructed paleomagnetic record mentioned in the Muscheler et al. (2005) paper, and that are discussed here are: 1. The discrepancy in the timing ofthe maximum geomagnetic field dipole strengths (the “virtual axis dipole moment”,VADM) during the late Holocene traced by the Be versus C records (see Fig. 6 inM05). 2. The interval of 18,000–34,000 years ago, where both Be and Cl VADM re-constructions show increased modulation compared to the remanence records, and the C data are poorly understood. 3. An anomaly 48,000 years ago where both Be and Cl showed increased modulation compared to the VADM record, and an anomaly58,000 years ago where only Cl shows increased modulation.Cosmic ray fluxes at Earth vary with the size and properties of the heliosphericmodulation region. A second simple change in the CISM properties, that would neces-sarily affect cosmic ray fluxes at the Earth, would be spatial variations in the ionizationlevels of the interstellar gas. At 1 AU the cosmic-ray fluxes below ∼
200 MeV/nucleonare dominated by anomalous cosmic rays (ACRs). For instance, the ACR oxygen in-tensity of 10 MeV/nucleon is an order of magnitude larger than for GCR oxygen(Leske et al. et al. et al. C, Be, and Cl productionchange as the ACR component vanishes? Three possible effects may contribute. Thefirst is that C is formed by thermalized neutrons near the top of the atmosphere,and the long storage time ( > . et al. C because of the longexposure times of nitrogen and carbon compounds in the atmosphere compared to thedirect production of cosmogenic isotopes formed by spallation. An opposite effect ofACRs on cosmogenic isotope production is suggested by the yield functions of Be and C as a function of the energy of the incident cosmic ray proton, which are shown inFigure 6 in Usoskin (2008). The production of Be is relatively more efficient at ACRenergies then the production of C, based on comparisons of the ratios of productionat ∼
200 MeV (ACR energies) and ∼
20 GeV (GCR energies). These ratios for Beand C are ∼ .
03 and ∼ . , respectively. These yield functions would then suggestthat the production of C is less sensitive to the ACR component of cosmic-ray fluxesthan is the production of Be. A third wild-card possibility is that rapid variations ofthe geomagnetic field may reduce coupling between the ACRs and the radiation belts,so that ACRs have the same access to the atmosphere as GCRs, reducing any effectof storage of ACRs in the radiation belts. In the absence of a detailed understandingof the effect of ACRs on cosmogenic isotope production, the discussions below linkingcloud transitions to the geological radioisotope data are highly speculative.4.1 Late Holocene Discrepancies and Anomalous Cosmic RaysThe VADM paleomagnetic field reconstructed from Be records peaks at 2,000 yearsBP, which is one millennium after the peak determined from C at 3,000 years BP (Fig.6 in M05). At the CHISM velocity, 3,000 years corresponds to a distance of 0.08 pc,which is a plausible value for the distance to the cloud surface in the α CMa direction.The 2,000-3,000 time interval is within the uncertainties of the solar entry into the LIC as inferred from the α CMa sightline by Frisch (1994), based on a filamentary structurefor the LIC. A LIC filamentary structure would be consistent with Figure 1, right, onlyif the density structure of the ISM moving at the LIC velocity is inhomogeneous.Since VADM variations anticorrelate with radioisotope fluxes, the C fluxes de-creased a millennium before the Be fluxes decreased in the Holocene. Muscheler et al. (2005) postulated that this discrepancy was due to the carbon cycle, because the Beand archeomagnetic field determinations generally agree.If the discrepancy is due, instead, to variations in the ISM, we suggest that ACRsmay be the culprit. The three possible influences of ACRs on the cosmogenic isotoperecord leads to three different explanations for the anomaly. Prior to entering the CISM,the Sun may have traversed a fully ionized cloud interface with no neutrals, no pickupions or ACRs, with a larger relative Sun-cloud velocity leading to higher interstellarram pressures (see Fig. 2 in SF08), and with reduced mass-loading of the solar wind bypickup ions. (1) If the enhanced yield of Be production at low energies is important,the absence of ACRs at 1 AU would increase C relative to Be, which is not seen.(2) An opposite result follows if the long exposure times for C formation from ACRsin the radiation belts is important, in which case an ionized ambient ISM would prefer-entially reduce C levels because of the concordant lack of C enhancement from themultiplying effect of the storage of ACRs in the magnetosphere. Once the Sun entersthe main part of the cloud, which is ∼
75% neutral, the low energy ACR componentis restored and the balance between the synchronization of the Be, Cl, and Cformation was restored along with the ACRs. (3) For the third alternative, if disrup-tion of the terrestial magnetic field affects the C production only, Be would staycoupled to the geomagnetic field while the timescale for C dating would be disrupted.The tight coupling between Be and the geomagnetic field suggests that the overallheliospheric modulation of the GCR fluxes did not change, so that the discrepancycould be due to the effect that the disrupted geomagnetic field has on the flux of ACRsinto the atmosphere.4.2 Cloud CrossingsDuring the period 18,000–34,000 years ago, both the Be and Cl VADM recon-structions deviate from the archeomagnetic remanence records. The increased VADMreconstructed from Be and Cl suggests reduced fluxes below that explained bythe archeomagnetic record, or extra heliosphere modulation of GCRs. Muscheler et al. (2005) suggested that possible changes in levels of solar magnetic activity may be re-sponsible for this difference. We speculate that this interval instead corresponds to anencounter between the Sun and slower clumpy ISM beyond the CISM in the downwinddirection that forms the second clouds observed towards α CMa (the Blue cloud, Figure1 right) and towards α CMi (the Aur cloud in Redfield and Linsky 2008), both within3.5 pc. Cloud velocities alone affect the GCR modulation because the heliosphere di-mensions increase as the ram pressure of the ISM decreases. The decreased Be and Cl fluxes in this interval would then suggest a larger heliosphere modulation region,that still contains a solar wind mass-loaded with pickup ions from interstellar neutrals.Cloud-Sun relative velocities of 26 km s − today, versus 21–22 km s − for χ Ori and α Aur, and 14 km s − for the Blue cloud towards α CMa (Table 1), suggest that cloudram pressure could have been a factor of ∼ − a larger heliosphere and larger GCR modulation region that are consistent with thedecreased fluxes found by M05.Additional discrepancies, about 46,000–48,000 and 58,000 years ago, are seen be-tween the radio isotope paleomagnetic and remanence paleomagnetic records. If theSun entered the neutral portion of the LIC ∼ ,
500 years ago, as indicated by χ Ori located in the downwind direction (Table 1), the VADM discrepancy suggests thatincreased cosmic ray modulation in the LIC, or decreased ACR production, could haveproduced the discrepancy. A slightly higher density, n (H ◦ ) ∼ .
25 cm − , would matchthe solar entry into the LIC to 48,000 years ago discrepancy. Comparisons between the structure and kinematics of interstellar clouds near the Sunand the paleomagnetic records deduced from radio isotopes such as Be, Cl, and C,are based on scanty knowledge of details of the three-dimensional spatial distributionand configuration of nearby interstellar clouds. Volume densities of both neutrals andelectrons in the nearest interstellar clouds are required to improve maps of the ISMdistribution. Some insight could be gleaned from photoionization models, as done forthe CISM; however high-sensitivity ultraviolet data are required for realistic models.The most significant open question is whether ACR variations are sufficient torequire consideration as a separate component that contributes to the production ofthe cosmogenic isotopes, and that varies separately from the GCR component as theheliosphere is modulated by the ISM. One aspect of answering such questions will be toexpand calculations of the fluxes of galactic and anomalous cosmic rays at the Earthbeyond the limited number of heliosphere configurations now available. More studyof the roles of ACRs versus higher GCRs as source populations of the radio isotopesare needed, since differences between the C, Be, Cl records are mandated ifACRs are a factor in C production rates. A better understanding of the physicalproperties and configuration of ISM close to the Sun is also required for accuratecomparisons between the timelines of the cosmogenic isotope and heliosphere boundaryconditions. Nevertheless, we conclude that in principle the attenuation of GCRs byan ISM-modulated heliosphere, and perhaps the presence or absence of ACRs, arecapable of accounting for differences between the paleomagnetic record determinedfrom cosmogenic isotopes versus remnance data.
Acknowledgements
PCF thanks the International Space Sciences Institute in Bern, Switzer-land, for hosting a stimulating meeting on the relation between cosmic ray fluxes and theterrestrial radio isotope record. This research has been supported in part by NASA grantsNNX09AH50G and NNX08AJ33G to the University of Chicago, and by the IBEX mission asa part of NASA’s Explorer Program. PCF would like to thank Ken McCracken and Jurg Beerfor helpful discussions.
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