New insights on geomagnetic storms from observations and modeling Page: 3 of 7
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STORM MAIN PHASE
One of the main mechanisms for rapid inward transport and energization of plasma sheet particles into the ring
current during the storm main phase is the development of a strong convection electric field lasting for several hours
(e.g., ). Such a large-scale convection electric field extending from dawn to dusk across the nightside
magnetosphere leads to the sunward motion of magnetospheric plasma injected far downstream in the magnetic tail.
Plasmasheet particles are transported due to this ExB drift into regions of stronger magnetic field and are energized.
To obtain the full plasma motion, one must add yet the corotation electric field caused by the Earth's rotation,
important for plasma residing close to the Earth. In addition to the electric drifts of charged particles are drifts
resulting from the gradient of the magnetic field strength and the curvature of the magnetic field lines. In contrast
with the electric drifts, magnetic drifts are energy dependent, therefore they are negligible for low energies -1 eV
(thermal plasma) and they are dominant for high energies ~l MeV (radiation belt particles). For ring current
particles at intermediate energies (~1-200 keV), both the electric and magnetic drifts have comparable importance in
the determination of the particle trajectories. Furthermore, since the gradient-curvature drift is charge dependent,
ions drift westward around the Earth, while electrons drift eastward and a current is formed around the Earth called
the ring current. Finally, we should note that over long periods of time high energy particles can diffuse radially as a
result of resonant interactions with the fluctuating components of the convection . This interaction is described as
radial diffusion because it results in transport of particles across the dipolar-like magnetic field lines in radial
To study the stormtime transport and acceleration of energetic particles, a kinetic ring current - atmosphere
interactions model (RAM) was developed by Jordanova et al. [8, 9]. The phase space distribution function in RAM
is defined for variables that are accessible to direct measurement, i.e., energy, pitch angle, radial distance in the
equatorial plane and magnetic local time (MLT). The model thus solves the bounce-averaged kinetic equation for
H, O', and He+ ions with kinetic energy from -100 eV to 400 keV and pitch angle from 00 to 90*. A region in the
equatorial plane spanning radial distances from 2 RE to 6.5 RE and all magnetic local times (MLT) is included. The
inflow of plasma from the magnetotail is modeled according to the total ion flux measurements from the
Magnetospheric Plasma Analyzer (MPA) and the Synchronous Orbit Particle Analyzer (SOPA) instruments on the
geosynchronous Los Alamos National Laboratory (LANL) satellites and the ion composition ratios are inferred from
the work of Young et al. . In recent work Jordanova et al.  used this model to address the processes of ring
current formation during the 22-23 April 2001 and 24-26 October 2002 storms of similar strength (Dst index
reaching minimum values of about -100 nT) but different solar origin. The interplanetary observations during 22-23
April 2001 from the instruments on ACE spacecraft indicated a relatively smooth south-to-north BZ excursion of the
IMF characteristic of a CME, leading to a gradually increasing interplanetary electric field (IEF) and monotonically
decreasing Dst. In contrast, the interplanetary medium on 24 October 2002 showed a stream-stream interaction (a
HSS overtaking a slower stream) during which the southward BZ component of the IMF was highly fluctuating,
leading to high temporal variations of the IEF and a step-like decreasing Dst.
We simulated ring current development during these two large geomagnetic storms and investigated the effect of
magnetospheric convection with our RAM using three different electric field formulations. In Figure 1 we compare
results from (1) a Kp-dependent Volland-Stem (V-S) electric potential model [12, 13, 14]; (2) a Volland-Stern
model including a potential drop from subauroral polarization streams (SAPS) , and (3) the UNH-IMEF model
[16, 17] driven by interplanetary conditions. The analytical V-S model predicts the largest electric potential during
the main phase of the storms when maximum Kp is observed, at hours -39 to 41 during both storms (Figure 1, top).
Including SAPS disturbance effects makes this model stronger and more realistic, creating a day-night asymmetry
and skewing the potential in the postmidnight sector as seen in self-consistent electric field model simulations [e.g.,
18]. The UNH-IMEF model is derived from electric field data primarily from the Cluster satellites. This electric
field data set is sorted according to several ranges of the IEF values measured by ACE and includes statistical results
from ground radars and low altitude satellites inside the perigee of Cluster (4 RE). Its magnitude increases for larger
IEF values, which occur during the main phase of the storms when IMF B, maximizes, at hour -34 during the April
2001 storm (Figure la) and at hour -36 during the October 2002 storm (Figure le). The plasma sheet ion density
from the MPA (Figures lb and if) are plotted along the nightside orbit of the LANL satellites (between MLT=18
and MLT=6) and exhibit temporal as well as spatial variations. The data indicate that the ring current source
population is highly variable throughout the intervals. Enhanced density is observed during the main phase of both
storms with peak values from -1.5 to 2 cma during April 2001 and from -1 to 1.5 cm3 during October 2002. The
ring current injection rate calculated with RAM and defined as the total energy gain per hour (Figures Ic and Ig)
reflects the variations of the convection potential and the inflow of plasma at the nightside boundary. There is
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Jordanova, Vania K. New insights on geomagnetic storms from observations and modeling, article, January 1, 2009; [New Mexico]. (digital.library.unt.edu/ark:/67531/metadc933491/m1/3/: accessed June 18, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.