A nearly continuous observation of the equatorial plasmasphere from the inner radiation belt to near a magnetopause reconnection site
Kevin J. Genestreti, Stephen A. Fuselier, John C. Foster, David Malaspina, Sarah K. Vines, Rumi Nakamura, James L. Burch
GGEOPHYSICAL RESEARCH LETTERS, VOL. ???, XXXX, DOI:10.1002/,
A nearly continuous observation of the equatorialplasmasphere from the inner radiation belt to near amagnetopause reconnection site
K. J. Genestreti, * S. A. Fuselier,
J. Goldstein,
J. C. Foster, D.Malaspina, S. K. Vines, † R. Nakamura, J. L. Burch Space Science Center, University of NewHampshire, Durham, NH, USA Space Science and Engineering Division,Southwest Research Institute, San Antonio,TX, USA Department of Physics and Astronomy,University of Texas San Antonio, SanAntonio, TX, USA Haystack Observatory, MassachusettsInstitute of Technology, Westford, MA, USA Laboratory for Atmospheric and SpacePhysics, University of Colorado Boulder,Boulder, CO, USA
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Key Points. ◦ We report a nearly continuous observationof the equatorial plasmasphere and plumeby VAP and MMS ◦ The proton temperature increases by a fac-tor of ∼
100 from the inner to outermostextent ◦ The density scales by ∼ L − and decreasesby a factor of ∼ Applied Physics Laboratory, JohnsHopkins University, Laurel, MD, USA Space Research Institute, AustrianAcademy of Sciences, Graz, Austria * Formerly at Southwest ResearchInstitute, San Antonio, TX, USA † Formerly at Southwest ResearchInstitute, San Antonio, TX, USA
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X - 3 spheric total electron content. The propertiesof the equatorial plasmasphere change dramat-ically from its the inner radiation belt to itsoutermost boundary (the magnetopause, neara reconnection site). The density decreases bya factor of ∼ L -shell as L − . ± . , in good agreementwith with theoretical expectations of the ex-pansion of a flux tube volume during outwardradial transport. The proton temperature in-creases by a factor of ∼
100 over this same range,with the most pronounced heating occurringat
L >
7, which was covered by the orbit ofMMS.
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1. Introduction
The plasmasphere is the cold and dense extension of the upper ionosphere into the mag-netosphere. The plasmasphere is distinguished from other coupled and often overlappingmagnetospheric plasmas by its high density (100s to 10,000s cm − ) and low temperature(0.1s to 10s eV). Owing to its low temperature, plasmaspheric dynamics are governed al-most entirely by (cid:126)E × (cid:126)B drift. The main plasmasphere torus is typically contained withinclosed (cid:126)E × (cid:126)B -drift paths at lower L-shells Grebowsky [1970];
Lemaire and Gringauz [1998].When the solar wind magnetic field points southward, magnetic reconnection allowsthe solar wind electric field to fractionally penetrate the magnetosphere, opening previ-ously closed (cid:126)E × (cid:126)B -drift paths and redirecting the previously trapped plasma sunward,forming a plume Grebowsky [1970];
Sandel et al. [2001];
Goldstein and Sandel [2005]. Theplasmaspheric plume is frequently observed at the noon-to-duskside reconnecting magne-topause [e.g.,
Walsh et al. , 2014], though the impact on reconnection of this dense andcold plasma remains uncertain. Unresolved topics related to the impact of the plume ondayside reconnection include, but are not limited to: modification (or not) of (1) the localreconnection rate
Borovsky et al. [2008];
Borovsky [2013];
Wang et al. [2015];
Fuselier et al. [2017], (2) the global reconnection rate
Lopez et al. [2010], (3) both the local and globalreconnection rates
Zhang et al. [2016], (4) the local fields geometry
Malakit et al. [2013],diffusive scale sizes, and redistribution of magnetic to thermal energy
Wang et al. [2014];
Toledo-Redondo et al. [2015, 2016];
Toledo-Redondo et al. [2016], and/or (5) downstreamexhaust speed
Walsh et al. [2013, 2014];
Fuselier et al. [2017].
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Scaling relations between density, temperature, and geocentric distance reveal that theplasmaspheric plume is warmer and less dense than the main plasmasphere torus.
Chappell [1974] first established an L − theoretical scaling law to describe the evolution of thedensity within a lossless flux tube radially transported in a dipole field. Statistical studieshave since used this relation to discriminate between the plasmaspheric plume and thetrough Sheeley et al. [2001];
Walsh et al. [2014]. Within the plasmasphere main torus, thedensity and temperature are inversely related
Comfort et al. [1985];
Comfort [1986, 1996].
Genestreti et al. [2017] found a positive correlation between the plasmaspheric protontemperature and the density of ring current ions within the plasmasphere main torus,which they tentatively attributed to wave particle heating. Based on statistical analysisof Time History of Events and Macroscale Interactions during Substorms (THEMIS) data,
Lee and Angelopoulos [2014] found that plasmasphere-like ions at large radial distances(
R > R E ) were generally hotter in the afternoon sector than elsewhere, which theyattributed to heating within the plume. Additional observations of plume ions at themagnetopause have showed that the plume is significantly hotter and more energetic thanthe plasmasphere main torus.In this study, we derive scaling relations between the density, temperature, and L -shell location of the plasmasphere. Unlike the aforementioned studies, we derive thesescaling relations for one single event, 22 October 2015, when NASA’s Van Allen Probes(VAP) and Magnetospheric Multiscale (MMS) missions provided simultaneous and nearlycontinuous coverage of the full radial extent of the equatorial duskside plasmasphere andplume. Data from one of the several MMS magnetopause encounters on 22 October 2015 D R A F T August 13, 2018, 12:56am D R A F T - 6
GENESTRETI ET AL.: PLASMASPHERE TEMPERATURE AND DENSITY have been used to examine the micro-scale influence of the cold plume ions on daysidereconnection
Toledo-Redondo et al. [2016]. For the first time, we use in situ data froma single event to track the temperature and density of the equatorial plasmasphere fromthe proton radiation belt, to its outermost extent, the magnetopause.In the following section, we describe the data used in this study and the means bywhich we determine densities and temperatures. In Section 3, we detail the state of theplasmasphere during the 22 October 2015 conjunction event and derive scaling relations,which show a factor of 1000 decrease in the density and a factor of 100 increase in thetemperature from the plasmapause near L ≈ L ( − . ± . , which is within error bars of the theoreticalscaling relation of Chappell [1974]. In the final section, we summarize our results.
2. VAP and MMS data
VAP consists of two probes, one leading and one trailing, which have apogees at nearlyidentical MLT and geocentric distances of 5.8 R E , with roughly 2 hours of orbital phasedifference Mauk et al. [2013]. MMS is a four spacecraft constellation with each probeseparated by tens of km
Burch et al. [2015]. During the first phase of the mission, MMSwas in an inclined equatorial orbit with an apogee and perigee at geocentric distances of12 and 1.1 R E , respectively Fuselier et al. [2016].We use data from VAP-A, the leading probe, as it crossed the duskside plasmasphereat low L -shells at nearly the same time MMS crossed the duskside plasmaspheric plumeat high L -shells. During this time, VAP-A and MMS remained within roughly 5 hours ofeach other in magnetic local time (MLT), as can be seen in Figure 1. We choose to use D R A F T August 13, 2018, 12:56am D R A F T
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X - 7 data from MMS-1, though the large-scale thermal and density structures that we examinelook essentially identical at each of the four spacecraft.VAP Helium Oxygen Proton Electron (VAP-HOPE) instrument
Funsten et al. [2013]and the MMS Hot Plasma Composition Analyzer (MMS-HPCA)
Young et al. [2016] aretop hat electrostatic analyzers with time of flight sensors. HOPE and HPCA measurethree-dimensional mass-per-charge-discriminated plasma ion distribution functions every22 and 10 seconds, respectively. HOPE measures directional fluxes at 72 logarithmicallyspaced energy steps from 1 eV/q to 50 keV/q. HPCA measures directional fluxes at63 energy steps logarithmically spaced between roughly 1 eV/q and 40 keV/q. HOPEmeasures the ion species of H+, He+, and O+ and HPCA measures H+, He+, He++,and O+. The low-energy threshold of HOPE is increased to ∼
25 eV/q during its perigeemode, which is used below roughly
L < .
5. During its perigee mode (roughly L ≤ Goldstein et al. [2014];
Sarno-Smith et al. [2015, 2016]. The sample rate of HOPE is not spin-synced, which leads toan oscillatory “beating” between the directions of the oversampled portion of phase spaceand spacecraft motion
Genestreti et al. [2017]. We do not account for the “beating” inthe HOPE data, as its effect on the temperature is minimal compared to the temperaturevariations across the full span of the plume). Unlike VAP, MMS has active spacecraftpotential control (ASPOC), which, when active, limits the potential of MMS to ≤ Torkar et al. [2016].
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Additional sources of data used in this study are (1) plasma densities derived fromMMS upper hybrid wave observations and continuum radiation cut-off observations madeby the MMS1 electric fields double probe data
Lindqvist et al. [2016];
Ergun et al. [2016],(2) densities derived from the spacecraft potential of VAP-A, (3) spacecraft potentialmeasurements from both MMS-1 and VAP-A, and (4) Total Electron Content (TEC) ofthe F-region ionosphere derived from GPS. Median values of the ionospheric TEC weremapped to the equator from 2 ◦ × ◦ grid cells at an assumed altitude of 350 km. Themapping was performed using the T04 model of the geomagnetic field Tsyganenko andFairfield [2004].As in
Genestreti et al. [2017], we use a 1-dimensional Maxwellian fitting algorithm todetermine temperatures from HOPE and HPCA omnidirectional flux data, since the bulkenergy of the plasmasphere is typically below the effective low-energy threshold of HOPEand HPCA ( ≥ ≤ (cid:126)E × (cid:126)B drift velocity inthe frame of the moving spacecraft. The (cid:126)E × (cid:126)B velocity is approximated using a centereddipole magnetic field and a Volland-Stern potential field that is parameterized by thesolar wind electric field Volland [1973];
Stern [1975].
Genestreti et al. [2017] noted thateven very large (100s of percent) errors in the approximation of the magnetic field do notaffect the resulting fit-determined temperature.
Genestreti et al. [2017] found that thedipole-approximated and measured magnetic field strengths differed by ∼
25% on average
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X - 9 for their event. We use the Volland-Stern approximation rather than the data from VAP’sdedicated set of electric field probes
Wygant et al. [2013], as small-amplitude and quasi-static electric fields are typically difficult to determine with reasonable accuracy. Notealso that (cid:126)E = − (cid:126)v × (cid:126)B cannot be determined since the bulk velocity of the cold plasmacannot be accurately determined. We use χ minimization to find the best-fit values ofthe density and temperature (as well as their associated 2 σ uncertainties) for each time-dependent measurement of the phase space density. Prior to applying the fit, we usePoisson uncertainty to assign a confidence level in each energy-dependent phase spacedensity value.
3. Case study of 22 October 2015
Figure 1 shows the orbits of MMS-1 and VAP-A for 12 hours following 22 October2015 16:00 UT, as well as the equatorial-projected ionospheric TEC, which may be usedto identify plasmaspheric plumes
Foster et al. [2002];
Walsh et al. [2014]. The TECwas calculated over the 20-minute interval 16:00–16:20 UT. After this period, differentialrecombination at conjugate points in the northern and southern ionospheres preventedone-to-one mapping of the ionospheric TEC to the equator. Based upon this single TECmap, it appears that MMS skirted the duskward edge of a much larger plume for 4 hoursfrom 16:30 to 20:30 UT. This is consistent with the observations of MMS, which showlarge densities (1/cc ≤ n ion ≤ f H + ( E ≤ (cid:29) f H + ( E > n H + /n tot ∼ D R A F T August 13, 2018, 12:56am D R A F T - 10
GENESTRETI ET AL.: PLASMASPHERE TEMPERATURE AND DENSITY were statistically determined from THEMIS data
Lee and Angelopoulos [2014]. Moderategeomagnetic activity was observed throughout the conjunction and the Kp index, whichis inversely correlated with the plasmasphere temperature
Comfort [1986], remained ator higher than 6. A sharp 500 nT enhancement in the auroral electrojet also occurred atroughly 18:45-19:15 UT.For four hours prior to its entry into the magnetosphere proper (roughly 14:30 to 16:20UT), MMS entered, exited, and reentered what was very likely a reconnection bound-ary layer at the magnetopause. This is evidenced by a long duration of simultaneousobservations of sparse and hot magnetospheric-like ions, dense and cold ionospheric-likeions, and dense and warm magnetosheath-like protons and alpha particles. The maximumshear model
Trattner et al. [2012] predicted that MMS was within 1 R E of the daysidereconnection site near 16:20 UT.The proton temperatures and densities from MMS and VAP are shown in Figure 2.For MMS, the fit-determined temperature and the temperature from the standard mo-ments integration (which has been calculated in the energy range E ≤
100 eV) are nearlyidentical, as is shown in 2b. The fit-determined density is lower than the waves-deriveddensity, indicating that the peak of the density profile was not fully captured by the fittingalgorithm. For VAP, we do not compare the fit-determined and standard temperatures,as the density ratio indicates that the vast majority ( ≥ σ uncertainty in the HPCA fit-derivedtemperature ( ± D R A F T August 13, 2018, 12:56am D R A F T
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X - 11 fit-derived density was 4.5 cm − or ± σ uncertainties for both thedensity and the temperature have been shown explicitly in Figures 2e-f (median values are2 σ T /T = 11% and 2 σ n /n = 36%). As discussed in Genestreti et al. [2017], the fitting algo-rithm is reasonably successful at determining temperatures for Maxwellian-like plasmas,but very poor at determining densities, especially when only the tail of the distributionfunction appears in the effective energy window of the instrument.As discussed in
Genestreti et al. [2017], there is a near-regular oscillatory signature inthe low-energy portion of the proton energy-intensity-time spectrogram from VAP-HOPE(see Figure 2d). The peaks / troughs in the intensity correspond to an anti-alignment/ alignment between (a) the portion of phase space that is over sampled by HOPE and(b) the direction of the bulk velocity of the plasma in the spacecraft frame. This beatingis strongly pronounced in the fit-determined densities and temperatures (Figures 2e-f),causing a peak-to-median difference in the temperature of 20%. The amplitude of theseoscillations increases near perigee as the spacecraft speed increases
Genestreti et al. [2017].(For the 22 October event, VAP-A reaches perigee after 17:30). Though this effect shouldcertainly be accounted for in any study of smaller-scale thermal structures within theplasmasphere main torus, we find that this ±
20% deviation is nearly inconsequentialcompared to the variations in the temperature that are observed across the full extent ofthe plasmasphere and plume.Figure 3 shows the relationships between the temperature, density, and L -shell locationsof MMS-1 and VAP-A. MMS-HPCA temperatures from 16:30 – 20:30 UT are from thestandard moments moments integration (over E ≤
100 eV), rather than the fit derived
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GENESTRETI ET AL.: PLASMASPHERE TEMPERATURE AND DENSITY temperatures, though the two temperatures were nearly identical. For VAP, temperatureswere derived from Maxwellian fits to the HOPE time-dependent fluxes between 16:00 –17:30 UT. After 17:30 UT, VAP-HOPE entered its perigee mode and the low-energythreshold was increased to 25 eV. Before 16:00 UT (near apogee), the ram energy of thespacecraft was small and the spacecraft potential was large compared to the low-energythreshold of HOPE, such that the bulk of the plasmaspheric protons were not observed.As such, it was not possible to extract reliable temperatures from the HOPE data. ForMMS, waves-derived densities were calculated from 16:30 – 18:45 UT. After this point,the upper hybrid line grew to frequencies that could not be measured by the electric fielddouble probes.We have computed rough exponential scaling relations for the proton temperature (Fig-ure 3a) and density vs L -shell (Figure 3b). First, we determined separate best fit linesfor the scaling between the temperature and L within the orbits of VAP and MMS. Thefactor of proportionality for VAP is –5.9 ± ± ± ± L changed from being largely constant to highly variable. For the scaling between thedensity and L -shell, we found a multiplicative scaling factor of 93000 ( ±
2% uncertainty)and a decay rate of –4.3 ( ± . L − scalingrate derived by Chappell [1974].
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As is shown in Figure 3, the properties of the plasmaspheric protons change dramati-cally from the base of the equatorial plasmasphere, near the inner radiation belt, to itsoutermost extent, the magnetopause. For this event, we observed a density decrease of ∼ × and a temperature increase of ∼ × . The temperature gradient is almostexclusively observed in the plume, which was observed by MMS. In the plume, there wasa significant population of higher-energy ( ∼
10 keV) protons observed along with the coldplasmaspheric protons, which may indicate that some form of cross-population interaction(e.g., collisional heating, wave-plasma interactions) is responsible for heating the plume
Gallagher and Comfort [2016].
4. Conclusions
We established scaling relations for the temperature, density, and L -shell for one event,22 October 2015, when MMS and VAP covered the entire equatorial plasmasphere andplume in a very nearly spatially continuous manner. An equatorial projection of the iono-spheric total electron content (TEC) suggested that MMS and VAP skirted the duskwardedge of the plasmasphere and plume. To calculate the proton temperature, we applied the1-d Maxwellian fitting scheme of Genestreti et al. [2017] to the VAP-HOPE and MMS-HPCA distribution function data. For MMS, the estimated and measured temperaturesare nearly identical, as the plume is sufficiently accelerated and heated to appear withinthe energy window of MMS. Earlier in the same 22 October orbit of MMS,
Toledo-Redondoet al. [2016] identified a unique diffusive scale size intermediate between the hot magneto-spheric ions and the electrons, which they associated with the presence of these cold ions.They also identified unique heating and acceleration mechanisms that occurred within
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GENESTRETI ET AL.: PLASMASPHERE TEMPERATURE AND DENSITY this cold ion diffusion region. Later in the orbit of MMS, we determined that these coldions, which may affect reconnection, were heated by a factor of 100 and the density wasdepleted by a factor of 1000 before even reaching the magnetopause. We found that theproton density scales as L − . ± . , which is very nearly identical to (within error bars of)the theoretical L − scaling derived in Chappell [1974]. Lastly, we noted that the vastmajority of the proton heating appeared in the plume at large L -shells ( L > L , which was the focusof this study. Acknowledgments.
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VAP-A
Figure 1: 20-minute median over 16:00–16:20 UT of the F-region TEC, which has been mappedto the equatorial magnetosphere using the T04 model. Black boxes indicate the locations ofMMS-1 and VAP-A at 16:00 UT. ‘X’-marks indicate the locations of MMS-1 and VAP-A at20:30 UT and 17:30 UT, respectively.
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X - 23 H + E ne r g y + S C P o t [ e V ] [ c m - s - s r - e V - ] H + T e m pe r a t u r e [ e V ] HPCA (moms) Maxwellian fit D en s i t y [ c m - ] HPCA (moms) HPCA (fit) Waves18:00 20:00hhmm
MMS-1 VAP-A
22 October 2015 22 October 2015 (a)(b)(c) (e)(f) H + E ne r g y + S C P o t [ e V ] H + T e m pe r a t u r e [ e V ] D en s i t y [ c m - ] hhmm Maxwellian fitHOPE (mom) HOPE (fit)SCPot [ c m - s - s r - e V - ] (e) (d) Figure 2: Overview of the 22 October 2015 conjunction event, where MMS and VAP providedsimultaneous and nearly continuous coverage of the duskside plasmasphere and plume. Observa-tions of the plume from MMS-1 are shown to the left and VAP-A observations of the plasmaspheremain torus are shown to the right. Proton energy-intensity-time spectrograms are shown on top,where the energies have been shifted upwards by the time-dependent electric potential of thespacecraft. Standard ± σ uncertainties in the fit-determined densities and temperatures, whichare determined during the χ minimization fitting process, are shown explicitly for VAP-HOPEby the dashed blue lines in panels (e) and (f). The standard ± σ uncertainties are not shownfor MMS-HPCA as they are very small (less than 5%). D R A F T August 13, 2018, 12:56am D R A F T - 24
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VAP-HOPE = (+), MMS-HPCA = (x) T e m pe r a t u r e [ e V ] D en s i t y [ c m - ] RBSP-EFW= (+), MMS-waves = (x) n = 92900 L -4.32 T = exp(-0.0055* L - 5.9) T = exp(0.24* L - 6.3) (a) (b) L -4.68 L -3.96 Figure 3: Scaling relations between the density (from MMS-EDP and VAP-EFW), temperature(from MMS-HPCA and VAP-HOPE), and L -shell. Fit relations are shown and listed in dark red,grey, and blue. The errors for the fit parameters are described in the text. The two light-bluedashed lines in (b) show the steepest and shallowest curves within ± σ error of the best-fit curve.error of the best-fit curve.