3. Observations and Statistical Results
Figure 1 displays the observations of proton fluxes during 16 - 30
March, 2015. The solar wind parameters and geomagnetic indices obtained
from the OMNIweb are plotted in Figures 1a-b. On 17 March 2015 when an
intense geomagnetic storm occurred, the solar wind speed jumped from
~400 to ~600 km/s and the interplanetary
magnetic field (IMF) Bz turned quickly from northward
(~15 nT) to southward (~-15 nT). The Dst
index dropped from ~30 to ~-220 nT with
the AE index increasing significantly from ~100 to 1500
nT. At the same time, the RBSPICE onboard the Van Allen Probe B observed
evident proton flux increases at energies of 44.7 and 81.6 keV (Figures
1c-d). However, the enhancements of 268.9 keV proton fluxes did not
occur until 18 March. The black solid lines in Figures 1c-e indicate the
plasmapause location calculated using Liu et al., (2015) model. In
Figures 1c-d, the proton fluxes outside the plasmasphere are higher than
those inside the plasmasphere during quiet times. During the recovery
phase after 18 March, 44.7 and 81.6 keV proton fluxes at lower L
(~2 – 4) gradually decreased while the 268.9 keV proton
fluxes increased and then remained relatively stable. Figures 1f-h
further show the proton energy spectra at the indicated time intervals.
As shown in Figure 1f, the proton fluxes generally decreased
monotonically with increasing energy over L ~3.0 – 5.6.
However, several days later (Figures 1g-h), the proton energy spectra at
L < ~5.2 exhibited “reversed” structures
with fluxes decay more significantly at low energies <
~200 keV, leading to flux minima at energies
~80 keV. These flux minima reduced with decreasing L
shells.
Figure 2 illustrates some key parameters of the reversed proton energy
spectra. This representative example occurred at 02:04:47 on 29 March
2015at L = 3.15. In this example, proton energy spectrum has a clear
flux minimum (f min) at Emin~82 keV and a flux maximum (f max)
at Emax ~221 keV with the ratio of flux
maximum to minimum reaching ~60. To automatically
identify the reversed proton energy spectra, we adopted three criteria:
(1) proton energy spectra show the existence of the maximum and minimum,
and the corresponding energy of flux maximum (Emax) is
greater than that of flux minimum (Emin), (2)\(\frac{f_{\max}}{f_{\min}}>3\), and (3) there must be at least one
energy channel between the Emax and Eminto avoid the misclassification.
To further investigate the relation between the geomagnetic activities
and the reversed proton spectra in a long period, we present the
geomagnetic indices and proton fluxes with three energy channels (44.7,
81.6 and 268.9 keV) during 2015 in Figures 3a-d with the white solid
lines representing plasmapause locations. When the Dst index suddenly
decreased and the AE index increased, the locations of plasmapause were
reduced to lower L (~2 – 3). For 44.7 and 81.6 keV
protons, the fluxes outside the plasmasphere are ~1
order of the magnitude higher than those inside the plasmasphere in most
cases, yet this is opposite to the fluxes of 268.9 keV protons. While
inside the plasmasphere, the proton fluxes at energies 44.7 and 268.9
keV are generally 1~2 orders higher than those at
energies 81.6 keV. With increasing energy, the proton energy spectra are
going to show the decreasing and then increasing trend which is the
reversed feature depicted in Figure 2. Figures 3e-h demonstrate the key
parameters of automatically selected reversed proton energy spectra. The
occurrence rate of the reversed proton energy spectra is calculated with
the grids of 0.1 L and one day. The locations of plasmapauses match well
with the upper boundaries of the region with the occurrence rate
> 90%. During quiet times, the plasmapauses usually locate
at L > 4.5 so that the reversed proton energy spectra
locate at L ~2 – 4. We find that the proton reversed
energy spectra are likely to be observed under active geomagnetic
conditions (Dst > -50 nT and the AE > 1000
nT). As shown in Figures 3f-h, the proton energies of flux maxima mostly
lie in the range of ~82-400 keV, decreasing with
increasing L shells. Similarly, the proton energies of flux minima for
reversed energy spectra are tens of keV. Note that there still are a few
events which distribute outside the plasmasphere at L >
~4.5 with Emax > 328 keV
and Emin > 100 keV. Figure 3f reveals that
the flux maxima are ~10 to 30 times greater than the
flux minima at L < 4, while the ratios decrease from
~10 to 3 with increasing L shells at L > 4.
Figure 4 shows the global distributions of reversed proton energy
spectra as a function of L shell and MLT for three indicated geomagnetic
conditions (Dst>-30 nT, -50 nT<Dst<-30
nT and Dst<-50 nT) from January 2013 to December 2016. From
top to bottom, each row presents the number of total samples, occurrence
rate, the corresponding energies of flux maximum and minimum, and the
ratio of flux maximum to minimum. The region of our interest has been
divided into smaller bins with the resolution of 0.5 L × 1 MLT, and the
blank bins means the observational samples are less than 50. Both the
Emax, Emin, and the ratio of flux
maximum to minimum in Figures 4g-o are valued by averaging the cases in
one bin. Most samples are observed during quiet times (Dst
> -30 nT) at L ~2.5 - 4.5 (Figure 4a). The
reversed proton spectra almost persistently exist over L
~ 2 – 4 during quiet times, with occurrence rates
>90%. Besides, the occurrence rates decrease with
increasing L shells, which is consistent with the observations shown in
Figure 1, and decreases under more active geomagnetic activities. The
upper boundary of higher occurrence rate (>90%) regions
shift from higher to lower L shells. Regarding to the MLT dependence, we
find that the occurrence rates on the dayside are slightly higher than
those of nightside, especially during the geomagnetically active periods
(Dst < -50 nT). In Figures 4g-l, the statistical distributions
of the Emax and Emin demonstrate that
the proton fluxes mostly reach the peaks at energies
~200 - 400 keV and drop to the valleys at energies
~50 - 100 keV. Both Emax and
Emin decrease first as the L shell increases to
~5, while they suddenly increase on two MLT sectors
(15-19, 22-05) at L>5 with relatively small samples. In
addition, the statistics of Emax and
Emin show a less geomagnetic activity dependence. The
ratios decrease with the increasing L shells, which are smaller under
the more active geomagnetic conditions (Figures 4m-o).