Extraordinary Solar Modulation Effects On Galactic Cosmic Rays Observed By V1 Near The Heliopause
aa r X i v : . [ a s t r o - ph . S R ] S e p EXTRAORDINARY SOLAR MODULATION EFFECTS ONGALACTIC COSMIC RAYS OBSERVED BY V1 NEAR THEHELIOPAUSE
W R Webber and and J J Quenby Department of Astronomy, New Mexico State University, Las Cruces, USA Blackett Laboratory, Imperial College London SW7 2BZ, UK
ABSTRACT
We discuss here two extraordinary increases of cosmic ray intensity that wereobserved by Voyager1 in the last 1.1 AU before it crossed the heliopause inAugust, 2012, at 121.7AU. These two increases are roughly similar in amplitudeand result in a total increase of ∼ ∼
2. During the 1 st increase the changes in themagnetic, B field are small. After the 1 st increase, the B field changes becomelarge and during the 2 nd increae the B field variations and the cosmic ray changesare correlated to within ± one day. The intensity variations of H and He nucleiand electrons during these time intervals are measured from 0.1 to over 1 GV.Thetotal increae of GCR in the two increases resemble those to be expected from asimple force field ”like” solar modulation with a modulation potential ∼
80 MV.This is nearly 1/3 of the total modulation potential ∼
250 MV that is requiredto produce the modulation of these particles observed at the Earth at the 2009sunspot minimum and adds a new aspect to the heliospheric modulation.
1. INTRODUCTION
When V1 crossed the heliopause on or about August 25, 2012 (day 238), there wereextraordinary changes in the magnetic field and the energetic particle intensities (Burlagaet al., 2013; Stone et al., 2013; Webber and McDonald, 2013). On that day, the particleintensities and field strength and direction began a change to values that have remainedrelatively unchanged now for over 20 months. Prior to this ”final” event there were severalunusual features in the energetic particle intensities that occured. For energetic particles, wemean GCR nuclei and electrons as well as the most energetic anomalous cosmic rays (ACR).The first of these intensity-time features occurred about May 7 th (day 128) when both the 2 –GCR nuclei and electrons increased by ∼
15% and 20% respectively. After reaching thesehigher levels near the end of May (day 150), these intensities remained nearly constant for ∼
58 days ( ∼ ∼
20% to a lower level where they also remained nearly constant for the 58day time period.On about July 28 th (day 210), the GCR nuclei and electron intensities increased suddenlyfor the second time. This increase was more rapid and eventually larger (20% and 40%respectively) than the 1 st increase. However, the increase occured in several stages, the finalone starting on August 25 th (day 238). At this time, GCR nuclei increased to their finalvalues which were ∼
32% higher than they were before May 7 th for >
70 MeV/nuclei and ∼
96% higher for 7-100 MeV electrons. The ” trapped” nuclei, termination shock particles(TSP) and anomalous cosmic rays (ACR), disappeared suddenly (Krimigis et al., 2013), sothat within just a few weeks the intensity of 2 MeV protons was less than 0.1% of theirintensity before May 7 th .The magnetic field changes, both in amplitude and direction, are a crucial backdrop forthe energetic particle changes. In this paper, we will discuss the GCR and magnetic fieldtemporal changes during the time period from day 128 to day 238 when V1 moved outward ∼
2. THE INTENSITY-TIME CHANGES AND A DISCUSSION OF THEIRIMPLICATIONS
The data presented here from V1 clearly shows two periods of increase for both GCRnuclei and electrons. This is illustated in Figure 1 which shows the integral rate of >
70 MeVnuclei and 5-60 MeV electrons. The total increase for each component from before May 7 th to after August 25 th (32% for >
70 MeV H and 96% for 5-60 MeV electrons) are made equalin the plot using different scales on the left hand and right hand axes. This is to show therelative magnitude of the increase on May 8 th to that in the second change between July28 th and August 25 th for the two species. The intensity changes for the two species are notidentical in relative magnitude or in their relationship to the magnetic field.The format in Figure 2 is similar to Figure 1 and shows the electrons 3-10 Mev (lefthand scale and the relative magnetic energy density ∼ B (right hand scale).For the 3-10 MeV electrons, the total increase is 69%, which is less than the increase of 3 – TIME
FIGURE 1 CN T S / SE C V15DR >70 MeV5DR 7-100 e MeV
100 200 300
Fig. 1.— Five day GCR running average of >
70 MeV (mostly nuclei, left scale), red lineand 5-60 MeV (mostly electrons, right scale), blue line. The two increases starting day 128and day 208 are the solar modulation events discussed in the text 4 –
TIME
FIGURE 2 CN T S / SE C B
100 200 300 B o o o Fig. 2.— Similar to figure 1 except the 3-10 MeV (mostly electrons) GCR are shown in red,along with the relative energy density ( B ) and direction of the magnetic field. 5 –96% for the 5-60 MeV electrons.For the B field, also shown in Figure 2, the changes in amplitude are over a factor 4 duringthis time period from values ∼ µ G to ∼ . µ G (the final field value) in just a few days fromday 208-210. There are two changes in B field direction (days 163-172) from a positive to anegative polarity and a much more sudden and final change, on days 208-209, from negativeto positive polarity.The 1 st step in the cosmic ray intensity changes at day 128 does not occur in associationwith any major B field amplitude change. After this intensity change and later during theperiod of constant GCR intensity, the B amplitude decreases and then increases by a factorof 3 between days 162-172, during a 10 day period when the field direction is also changingfrom 270 ◦ to 90 ◦ . During this 10 day period the field inclination increases from ∼ ◦ to ∼ ◦ (Burlaga et al., 2013). The locally measured GCR intensities were remarkably insensitive tothese extraordinary B field changes.The next field polarity change occured on day 209 and is the final, decisive polarity changefrom 90 ◦ to ∼ ◦ . One day later the B field amplitude changes by a factor 3 in one dayto its final value of 4 . µ G. The B field changes that occur between days 210 and 238 arematched within ± day by the corresponding increases in GCR and decreases in TSP andACR as seen in Figure 2 and also in Krimigis et al., (2013) and Burlaga et al., (2013). Also,as seen in Figure 2, the lower rigidity electrons are more responsive than the nuclei to thesechanges in field amplitude that pass V1 between day 210 and day 238.So overall we have the observation by V1 that the 1 st GCR increase stating on day 128 was not coincident with corresponding large B field amplitude or direction changes. A followingperiod of nearly constant GCR intensity, however, was coincident with large amplitude anddirection changes of the B field, as well as unusual field elevation angle changes.The 2 nd GCR increase starting on day 208 and the following intensity changes culminatingwith the final increase on day 238, were all simultaneous with ± one day with the very largeB field magnitude changes, but there were no field direction changes during this time. Thelower rigidity electrons were more responsive to these B field changes than the GCR nucleiwhose rigidity is ∼
50 times that of the electrons. In this second increase of GCR, the TSPand ACR intensity changes were opposite to the GCR to within ±
3. INTENSITY AND SPECTRAL CHANGES OF GCR H, He NUCLEIAND ELECTRONS BETWEEN DAY 128 AND DAY 238 OF 2012
In the Stone et al., (2013) article in Figures 2, 3 and 4 the intensities and spectra of Hand He nuclei and electrons are shown for the time periods before May 8 th and after August 6 –25 th . This includes the period of the two GCR increases. Below an energy of ∼
80 MeV/nuc.,the time period before May 8 th is contaminated by background ACR intensities for H andHe nuclei and therefore these energies cannot be used in the comparison.The intensity changes of these particles with different mass to charge ratio, A/Z, have histor-ically been very useful for understanding the origin of solar modulation effects. For example,Gleeson and Axford (1968) have compared the H and He intensity changes in their deriva-tion of the force field approximation to the solar modulation. They find that if the changesin intensity, M = βln ( j ( P ) /j ( P ), are plotted as a function of rigidity, there is a split-ting of the modulation for each species according to their charge to mass ratio A/Z. Thissplitting arises from the fact that the modulation itself, expressed in MeV, is defined by amodulation function, Ψ = Ze R (( V / dr/K ( r ) where K ( r ) equals the scaler diffusion coef-ficient which has dependence K ( r ) ∝ βP f ( r ) and V is the radial wind speed. Gleeson andAxford (1968), however, also introduced another quantity called the modulation potential, φ = R ( V / dr/K ( r ) expressed in MV. This modulation potential is the same for H, Heand electrons at the same rigidity. The modulation function, M, that is commonly used, isdefined by Ψ. M describes the amount of modulation between two different times (or places)and is different at the same rigidity for particles of different A/Z. Hence the use of the termcharge splitting when this quantity is used. The section by Ken McCracken pp 50-58 inthe book Cosmological Radio Nuclei (2012) discusses the two quantities, modulation func-tion and modulation potential. The modulation function is useful for comparing intensitychanges of different species.Figure 3 is a plot of the expected variation of βln ( j /j ) with P for H and He where thesolar modulation potential is taken to be 80 MV. Note the charge splitting of the amount ofsolar modulation which results in a 2 times greater modulation for H than He at rigidities ≤ th to after August 25 th .They are roughly consistent with an overall mododulation ∼
80 MV,but with no obvious charge splitting. The observed modulation function for electrons atlower rigidities is independent of P and is ∼ ∼ .
4. GENERAL COMMENTS
It is not the intent of this paper to develop a theoretical model for an explanation ofthese modulation effects observed by V1 by the CRS instrument. We believe that the GCRintensity changes are so unusual and unprecedented in the history of cosmic ray studies thatthey are not easily accomodated within the Parker (1963) heliospheric modulation picture 7 –Fig. 3.— The modulation function M = βln ( j /j ) calculated for a modulation potential=80MeV (solid lines) and the observed modulation obtained by comparing the H (black), He(red) and electron (blue) spectral intensities measured at V1 before day 128 and after day238 of 2012. The details are discussed in the paper. 8 –as developed by many others, e.g. Gleeson and Axford, 1968; Fisk and Axford, 1969. Butthere are certain features of the modulation that may indicate the characteristics of the Bfield, plasma flows, etc., that affect the entrance of the Local Interstellar Spectrum (LIS)cosmic rays into the heliosphere.We recognise that the 1 st step in this modulation process is related to the 1 st event thatstarted about May 8 th (DOY 128). This event contributed about 40% of the total increasefor both GCR nuclei at higher rigidties and electrons at lower rigidities. The changes inthe B field were small during the 10-20 day time period of the 1 st increase as noted earlier.During the following ∼
58 day time period up to about day 200 the intensity of the GCRremained relatively constant to within a few percent. However the B field recorded some ofthe largest changes yet seen at V1 along with a polarity from 270 ◦ → ◦ and along withunprecedented changes in the elevation angle near the end of this constant GCR intensityperiod.The lack of significant time correlation between the GCR intensity changes and the B fieldchanges during the entire time period from DOY 128-208 suggests that the GCR changesduring the first increase could be related to much larger scale features that may not be evi-dent in the local field being measured at V1 at that time.The lack of correlation between B and the GCR intensities in the 1 st event is definitelynot present in the 2 nd event which started on July 28 th (day 208), In this event, from July28 th to August 25 th , the GCR and B field changes were correlated to within ± ∼ th (day 238). In each of the increases theB field magnitude and direction at the times of the B field maxima was essentially the sameas that observed after August 28 th . The GCR electrons and nuclei, however, did not reachintensities that were observed after August 28 th . For electrons from 5-60 MeV the peakincreases were ∼
80% of the post August 28 th intensity. For nuclei, the increases reachedlevels ∼ −
60% of the post August 28 th intensity. So there is a distinct rigidity dependenceof the GCR distribution within these structures that pass V1. Note that the speed of V1 is ∼ .
01 AU/day. So the ± day correlation between B and GCR could have a scale ∼ . β.ln ( j LIS /j ) vs P data for this modulation of the different species, shown in Figure 3, areimportant: (1) The intensity changes of electrons are nearly independent of rigidity at lowrigidities. In addition, if the values of electron modulation function β.ln ( j LIS /j ) at lowrigidities is extrapolated to higher rigidities, it has a value ∼ ∼ /
5. SUMMARY AND CONCLUSIONS
This paper describes two large and unprecedented modulation events of GCR observedat V1 starting on May 8 th and July 28 th just prior to the crossing of the heliopause on August25, 2012 at a distance of 121.7 AU. These events resulted in the increase of GCR electronsfrom 5-60 MeV by a factor ∼ ∼ . ∼
80 MV which is 1/3 of the totalsolar modulation required to reproduce the spectra of the same nuclei observed at Earth ata time of sunspot minimum in 2009 (eg., Mewaldt et al., 2010). Thus a new and significantfeature is added to the description of solar modulation in the heliosphere.The first modulation event occured when V1 was 1.1 AU inside the HP. The intensity in-creases starting on May 8 th (day 128) and continuing up to day 150 amounted to about 40%of the combined increase for both events. During this GCR increase there were only modestchanges, both negative and positive, in the B field amplitiude with no change in direction.For the next 60 days the GCR and ACR intensities remained almost constant. However,between days 150-160 the B field changed direction from 270 ◦ to 90 ◦ and then decreased bya factor ∼ .
0, followed in a few days with a sudden increase by a factor ∼ ∼ ◦ to 90 ◦ . It almost seems like the B field was ’turningits self inside out’ in a period of a few days, perhaps due to the passge of a very large scalequasi-periodic structure, but without any observable effects on GCR or ACR.The time period of 90 ◦ polarity ended suddenly on July 28 th (day 208) when the polaritychanged to 270 ◦ followed by an increase in the magnitude ot the B field (on day 209), againby a factor ∼ ∼ . µG after day 238. This increase on July 28 th
10 –and the subsequent changes in B were coincident within 1 day with corresponding positiveand negaive intensity changes of GCR. In this period the changes in ACR (Stone et al., 2013)and TSP (Krimigis et al., 2013 ) were exactly opposite to those of GCR. The details of thesechanges provide a glimpse into features of the heliopause with structures with a scale whichcould be ∼ . af ter the increase and whenthe GCR changes themselves were small. The 1 st and 2 nd GCR increases have roughly thesame magnitude and same rigidity dependence, however, despite their greatly different cor-relation with the B field. They could be part of a larger structure ∼ ∼ .
6. ACKNOWLEDGEMENTS
W.R. Webber wishes to thank his Voyager colleagues from the CRS instrument, ProjectPI Ed Stone, Alan Cummings, Nand Lal, Bryant Heikkla and the late Franck McDonald.Support from JPL is greatly appreciated.
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