The large scale convection electric field, ring current energization, and plasmasphere erosion in the June 1, 2013 storm Scott Thaller Van Allen Probes SWG telecon July 10, 2015

Van Allen Probes investigation of the large-scale duskward electric field and its role in ring current formation and plasmasphere erosion in the 1 June 2013 storm S. A. Thaller, J. R. Wygant, L. Dai, A.W. Breneman, K. Kersten, C.A. Cattell, J.W. Bonnell, J.F. Fennell, Matina Gkioulidou, C.A. Kletzing, S. De Pascuale, G.B. Hospodarsky, and S. R. Bounds Thaller et al., [2015] is published in JGR, DOI: /2014JA Based on the paper:

Main Points (Introduction) During the strong geomagnetic storm on June 1, 2013 (min. Dst ~ −120 nT), a large scale convection electric field is observed, lasting at least ~7 hours, with enhancements ~1-2 mV/m, and extending down to L~2.3. We investigate the response to this electric field of the plasmasphere (erosion), and the ring current ions with energies keV. Note that lower energy ions (1-52 keV) are present, and do contribute to the ring current ion pressure, but not the focus of this study. The keV ions have undergone a greater amount of energization than the lower energy ions.

Main Points (Introduction) We show nearly simultaneous enhancements in the duskward convection electric field, plasmasphere erosion, and increased pressure of 58–267 keV ring current ions. The duration and intensity of the electric field enhancement is long enough to drive the plasmasphere erosion and transport the ring current ions, while energizing then to the observed energies, according to a simple calculation. These observations suggest that we are observing the electric field that energizes the ions and produces the erosion of the plasmasphere. Injection events also likely play an important role in ring current ion energization in this storm. There will be some discussion of these too.

E y = -V x B z E y solar wind -dDst/dt Dst (b) (c) (d) (e) Residual vxB fields around perigee (4 green shaded bars) E y MGSE E y MGSE (5 min. ave.) mV/m nT (a) nT/s The large scale convection electric field is ~14% of the solar wind electric field. Consistent with typical reconnection efficiencies. The storm main phase and enhancements in the current systems (mainly ring current and tail currents) occur during the electric field enhancement. An extended interval (at least ~7 hours) of enhanced (1-2 mV/m) duskward electric field. This extends over a spatial area of ~6 RE. Adapted from Thaller et al. [2015]

Van Allen Probe A, June 1, 2013 Out bound In bound Sunward Orientation of Van Allen Probe A’s orbit during the June 1, 2013 storm Adapted from Thaller et al. [2015]

Van Allen Probe A June 1, 2013 v mV/m kV Residuals near perigee Y MGSE Dst RE 30 R E Magnetopause Shue et al Van Allen Probe A orbit 5.5 R E x GSE (R E ) y GSE (R E ) Potential drop across orbit (30 kV) (a) (b) (c) (d) (e) (f) E y MGSE (corotating frame) 5 min. average Adapted from Thaller et al. [2015] The potential drop along the Y MGSE displacement during an orbit of Van Allen Probe A is extrapolated across the magnetosphere, and found to be 160 kV. This agrees with the cross polar cap potential estimated from Φpc = 20(Kp +1) [Heppner, 1977], with the storm Kp (max.) of 7.

Dst L shell Plasma density EyEy Inbound (pre-midnight to dawn) EyEy Outbound (post-noon to dusk ) Dst Plasma density E y MGSE (corotating frame) (mV/m) Plasma Density cm -3 Duskward electric field and cold plasma density as a function of L shell and time from May 29 through June 5, 2013

Adapted from Goldstein [2006] Adapted from Grebowsky 1970 Van Allen Probe A Orbit, June 1, 2013 General plasmasphere shape for steady Ey >0 Ey = 0.9 Lsp = 3.9

Dst L shell EyEy Inbound (pre-midnight to dawn) EyEy Outbound (post-noon to dusk ) Dst E y MGSE (corotating frame) (mV/m) keV Ion Pressure (nPa) ~ keV Ions (MagEIS) Duskward electric field and ( keV) ion pressure as a function of L shell and time from May 29 through June 5, 2013

Hot ion flows contours (A rough cartoon) E Y = 0 E Y > 0 When the large scale duskward electric field is enhanced the hot ion flow contours are asymmetric about the Earth-Sun axis. The orbit of a spacecraft will often be oriented such that the in and out bound passes are in regions with different flow characteristics. When the duskward electric field subsides the ions will drift symmetrically about the Earth. Enhanced ion pressures are (mainly) seen on in bound passes after main phase. This may be due to the ion drift path Korth et al. [2000] investigated ion response to enhanced E in different MLTs. Dawn Dusk

HOPE proton and O + pressures E y MGSE (corotating frame) (mV/m) ~1-52 keV protons Ion Pressure (nPa) L shell EyEy ~1-52 keV protons Outbound Inbound ~1-52 keV Oxygen EyEy Dst Ion Pressure (nPa)

Simple Model Calculation We used a simple model consisting of a dipole magnetic field and constant, azimuthally westward, electric field to calculate the earthward and westward drift of 90° pitch angle ions. This model is applied to determine how far earthward ions can drift while remaining on Earth's nightside (before reaching 18 MLT), given the strength and duration of the convection electric field.

Simple Model Calculation For a duskward/azimuthal electric field with an intensity of 0.9 mV/m (the average of the field observed), an ion starting at L = 8, MLT = 3 with an initial energy in the range of ~5-26 keV (not observed by RBSP) will drift earthward in 1-2 hours to locations between L~3.5- 5, and have final energies of ~ keV. This shows that such a convection electric field as that observed in the June 1, 2013 storm can take ions near the inner edge of the plasma sheet (if the enhanced E field exists there) and transport them to the location of the observed ring current, while energizing them to typical ring current energies.

58.4 keV Diff. ion flux MagEIS E y MGSE (co-rotating frame) Van Allen Probe A June 1, 2013 (a) (b) (c) (d) cm -1 s -1 sr -1 keV -1 nPa mV/m δP mag (variations > 20 min subtracted ) keV ions (MagEIS) Isotropic pressure MLT L hhmm Adapted from Thaller et al. [2015] Candidate Injection Events An interval observed on Van Allen Probe A near the end of the main phase during which there are ~ 5 injections (dashed lines) of ions in the keV energy range. Enhancements in the duskward electric field occur at the same time as the injections. These injections are accompanied by a sudden local pressure increase of ~1 nPa. The sudden pressure increases are accompanied by a simultaneous sudden decrease in the local magnetic pressure. This is likely due to MHD pressure balance (The HOPE 1-52 keV ions also show ~1 nPa increases).

Conclusions 1.) The simultaneous enhancements in the large scale duskward electric field, plasmasphere erosion, and ring current ion pressure, combined with a simple model calculation, indicate a direct role of the electric field in ring current energization and plasmasphere erosion. 2.) The range of L shell in which the most intense duskward electric field enhancements occurred was similar to that in which the ~ keV ring current ions had the highest pressure. This occurs: – On the pre-midnight to post midnight side after the convection electric field decreases; – On the post-noon to dusk side both during and after the convection electric field decreases.

Conclusions Other storms (March 17, 2013 and June 28-29, 2013) were examined, the results of those analyses were consistent with those of the June 1, 2013 storm. Injection events and associated increases in the local ion pressure do occur during the main phase of the June 1, 2013 storm, often in association with large duskward electric fields of ~10 mV/m, which are superimposed on the large scale convection electric field. The ions in these injections are located in magnetic cavities likely associated with MHD pressure balance. The typical duskward (“injection”) electric field intensities are ~10 mV/m and the ions have characteristic energies ~100 keV. Assuming the ions are energized by drifting through this electric field, a width of the injection region can be inferred to be ~1.6 RE. This width is consistent with the injection width of ~1.8 RE, determined by other means (arrival time of different energy ions) by Gkioulidou et al. [2014].

Extra slides

E y MGSE (corotating frame) (mV/m) ~ keV Ions (MagEIS) Dst keV Ion Pressure (nPa) L shell Plasma Density (cm -3 ) Plasma density EyEy Van Allen Probe A (outbound; post-noon to dusk ) (a) (b) (c) (d) Adapted from Thaller et al. [2015] The main phase, indicated by the vertical dashed line, is again characterized by a duskward electric field enhancement, erosion of the plasmasphere, and an increase in ion pressure. But in this case, the plasmasphere erosion and ( keV) ion pressure enhancements do not go to as low an L as the electric field enhancement. Note the asymmetries in Ey and the plasmapause location in L (compared to in-bound). The region of main and recovery phase higher pressure (>8 nPa) ions extends a similar range in L as does the most intense (Ey ≳ 1.5 mV/m) electric field during the main phase.

E y MGSE (corotating frame) (mV/m) ~ keV Ions (MagEIS) Dst keV Ion Pressure (nPa) L shell Plasma Density cm -3 Plasma density EyEy Van Allen Probe A (inbound; pre-midnight to dawn) (a) (b) (c) (d) Adapted from Thaller et al. [2015] The main phase, indicated by the vertical dashed line, is characterized by the duskward electric field enhancement, erosion of the plasmasphere, and an increase in ion pressure at lower L shell (<4). These enhancements all occur down to L ~ During the recovery phase the highest ( keV) ion pressure occurs between L ~ 3.7 and L ~ 5. It is interesting to note that this is roughly the range (L~3.5 to 4.8) over which the duskward electric field was the most intense during the storm main phase.