The role that the cleft/cusp has in ionosphere/magnetosphere coupling makes it a very dynamic region having similar fundamental processes to those within the auroral regions. With Polar passing through the cusp at 1 Re in the Spring of 1996, we observe a strong correlation between ion heating and broadband ELF (BBELF) emissions. This commonly observed relationship led to the study of the coupling of large field-aligned currents, burst electric fields, and the thermal O+ ions. We demonstrate the role of these measurements to Alfvenic waves and stochastic ion heating. Finally we will show the properties of the resulting density cavities

Chandler, Michael

Coffey, Victoria

Singh, Nagendra

NASA Technical Reports Server

On Alfvenic waves and stochastic ion heating
with 1Re observations of strong field-aligned currents, electric fields, and O+ ions
Victoria Coffey1, Michael Chandler1, Nagendra Singh2
1NASA Marshall Space Flight Center, 2University of Alabama in Huntsville
Abstract
The role that the cleft/cusp has in ionosphere/magnetosphere 
coupling makes it a very dynamic region having similar fundamental 
processes to those within the auroral regions. With Polar passing 
through the cusp at 1 Re in the Spring of 1996, we observe a strong 
correlation between ion heating and broadband band ELF (BBELF) 
emissions. This commonly observed relationship led to the study of 
the coupling of large field-aligned currents, bursty electric fields, and 
the thermal O+ ions. We demonstrate the role of these measurements 
2.  Ion Heating and Stochastic Ion Heating Rates 3.  Observations, Particles and Fields
For the dayside cusp, BBELF and ion energization is often 
coincident with large currents and large electric fields perp. to the 
local magnetic field. Figure 6 shows the Polar/EFI perpendicular 
electric field (red) coincident with the increase in TIDE O+
perpendicular temperature (blue)  for May 12, 1996.   Observations 
of continuous 90° O+ ion conics during this same time period 
indicate that a local ion heating mechanism is in effect (Figure 7).
Figure 9. Panel A.  High 
resolution perpendicular 
electric field, Ex, for May12.   
The correlation between ion energization and the BBELF power is 
well known. Figure 4 demonstrates this strong correlation as 
observed from Polar for the dayside cusp passes at 5100 km.   This 
data consists of » 6000 data points with the O+ perpendicular 
temperatures averaged within each PSD bin. The profile at the 5.62 
Hz frequency band is similar for the other frequencies profiles in the 
PWI BBELF range with no structure seen at the gyrofrequency in 
frequency profiles   The temperatures as a function of the sum of the 
Figure 9 shows the perpendicular electric fields with Alfvenic
fluctuations and the calculation of χ = δE/dx (ωOBo)-1>1. The 
observed electric field gradients do meet the stochastic ion heating 
mechanism criteria, χ > 1, for assumed wavelengths of 1921 meters 
(approximately 4ρO+ with 100 eV at 5100 km and lower).
to Alfvenic waves and stochastic ion heating. Finally we will show the 
properties of the resulting density cavities.
On a moving spacecraft, short wavelength dispersive Alfven Waves 
(DAW) are Doppler shifted and observed as ELF electromagnetic Fi 6 P l TIDE d EFI Fi 7 TIDE l it
Evidence of Alfvenic
fluctuations. 
Panel B,C.  Calculation of 
electric field gradients reveals 
χ >1 for two assumed 
wavelengths scales;  100 
meters (B) and 1921 meters 
(C). (O+ 100 eV Larmor radius 
= 473 meters.)
1.  Introduction
.
PSD from 5.6 to 311 kHz is also shown.  A threshold is suggested 
between 10–9 and 10–8 V2/m2/Hz and is analogous to the threshold of 
10–9 V2/m2/Hz at ~1500 km found by Knudsen [1998] during Freja
apogee passes.   
Although limited in sampling period, the comparison of three different 
heating rate calculations from measurements for stochastic ion 
acceleration for a static electric field are shown in Figure 5.   Each of 
the three rates are obtained using Polar measurements.   All assume 
local heating as observed in Section 3 of this paper.
5    Ob ti  D it  St t
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4.  Observations, Fields
noise (BB-ELF) over a broad frequency range (typically below 220 
Hz).  In this paper we follow the plasma properties of the O+ ion 
energization with the power in the BBELF waves.  In a  typical cusp 
pass at 5100 km, the Polar spacecraft observes the BBELF waves 
that are commonly coincident with ion energization. Figure 1 shows 
an example of a typical spectrogram of the BBELF noise from the 
Polar Plasma Wave Instrument (PWI) (bottom) along with the Polar 
TIDE O+ energization (top). A polar contour of the ubiquitous nature of 
BBELF from PWI in shown in Figure 2a.  The largest PSD is observed 
near 76.8° latitude where the peak O+ temperatures occur (2b).  The 
l it  t  d i   t i l   ill t ti  th  d it  
Figure 4. Strong correlation 
(R=0.997) of averaged O+
perpendicular temperature as a 
function of  the 5.6 Hz PSD 
(blue) and the sum of the 
frequency bands (red).
gure .  o ar  an   
observations showing  O+ perp. 
temperature increase coincident with 
large perpendicular electric field, Ex.
gure .    ve oc y 
distributions of 90 ° ion conics 
during full time
period.
When the perpendicular scale of DAW waves become comparable to 
the ion gyroradius the wave becomes dispersive and interacts strongly 
with the particles. Again, these short wavelength DAW are Doppler 
. serva ons, ens y ruc ures
It was suggested by Chaston [2006] that the width of the density cavities 
can increase due the production of Alfven waves on the transverse 
density gradients.  Over the narrow cusp passes, we averaged the 
depth, (n0 -n)/n0 ,, and the width of the density cavities for each PSD bin.  
Figure 10 suggests an increase in the density depth from 30% - 60% 
with increase in PSD.  Figure 11 also suggests an increase in the 
average width of the density cavity from 168 km to 294 km with increase 
in PSD.
Figure 1.  Top panel:  Polar 
TIDE energy/time 
spectrogram showing 
typical cusp ion O+ energy 
increase from 21:42 to 
21:45 in the cusp.  Bottom 
panel:  PWI observations of 
BBELF emissions at this 
same time.
ve oc y momen s ur ng a yp ca cusp pass us ra ng e ens y
cavities occuring on the temperature gradient are shown in Figure 3.
Figure 5. Comparison of heating 
rate calculations as a function of 
PSD.  TIDE/PWI cusp heating 
rate measurements (red) and 
(blue). Comparison to the heating 
rate for stochastic ion 
acceleration (black).
shifted and observed as ELF electromagnetic noise or
BB-ELF.  One method of heating the ions when ω < ωi can be 
explained by stochastic ion heating by these short perpendicular 
wavelength electrostatic waves [Chaston 2005].   Here we look at 
signatures of one typical cusp pass provided by Polar/TIDE and EFI 
and determine through data analysis if the fields are Alfvenic and if the 
ion heating can be explained by the stochastic process. The ratio of 
δE/δB was constructed for spacecraft frequencies below 
ωO+.  The  fluctuations are greater than the Alfven speed of » 104
km/s.  Figure 8 shows the power spectrum of δE and δB and their 
ratio as a function of frequency and wavelength in the spacecraft 
Figure 10. Average 
density depth as a function
Figure 11. Average width of 
density cavity as a function of2 ) 2b)
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eV/(cm2 s sr eV)
(V/m)2/Hz
6.  Summary
Cusp observations are often coincident with strong perpendicular electric field 
gradients and continuous 90° conics over the narrow cusp passes. We begin by 
showing that the power observed from BBELF, now thought to be Doppler-shifted 
short-wavelength DAW, is consistent with stochastic ion heating by turbulent fields. 
Analysis for a particular Polar pass show Alfvenic fluctuations evident in the 
perpendicular electric field and that the δE/δB fluctuations are greater than the local 
averaged Alfven speed and so the electromagnetic fluctuations are suggestive of 
Alfven waves.  Data analysis shows that the stochastic heating mechanism may be in 
ff t f  th   t 5100 k    Th  t h ti  h ti  it i   1  i  t f  l  
frame. 
     
of PSD
      
PSD.
Figure 8.
- 1st panel.  High resolution 
perp. electric field, Ex.  Evidence 
of Alfvenic fluctuations. 
- 2nd panel.  Power spectrum of 
electric, δE⊥1, and magnetic 
field, δB⊥2, limited to 8 Hz by 
sampling
Figure 3. Example of typical 
moments during the cusp passes. 
Figure 2a) Contours 
of PWI electric PSD 
at 10 Hz frequency 
band  2b) Invariant 
latitude as a function 
of O+ perp. 
temperature.  The 
mean invariant 
latitude is ~76.8°
a
Heating Rate (blue)= dT/dt ≈ V
p
⋅ E
x
Staciewicz [2000
Heating Rate (red)=  dT/dt =  
T
pf
−T
po
⎛ 
⎝ ⎜ 
⎞ 
⎠ ⎟ 
L
V
=
T
pf
−T
po
⎛ 
⎝ ⎜ 
⎞ 
⎠ ⎟ 
V ⋅Δt⎛ ⎝⎜ 
⎞ 
⎠⎟ 
65
70
75
80
85
90
0 10 20 30 40 50 60 70 80
I
n
v
a
r
i
a
n
t
 
L
a
t
i
t
u
d
e
 
(
d
e
g
r
e
e
s
)
Perpendicular Temperature (eV)
e ec or e cusp a m. e s oc as c ea ng cr er a, χ > , s me or sca e
lengths appropriate for O+. Although limited by the sampling period, analysis on the 
density cavitation shows vertical depths ranging from 30-60% and horizontal density 
cavity widths of ~ 200 km.
7.  References
. 
- 3rd panel. δE⊥1/δB⊥2 as a 
function  of frequency in the 
spacecraft frame.  Va = local 
Alfven speed averaged over the 
interval shown.
- 4th panel. If the waves are due 
to DAW we can  plot δE⊥1/δB⊥2
as a function of wavelength.  Vs 
= spacecraft velocity.
Lotko, JASTP, 2007;  Chang, JGR, 1986;  Chaston, JGR, 2005;   Chaston, JGR, 
2006, Staciewicz 2000
Viewing from equatorward to 
poleward side (right to left:   
Temperatures rise steeply on 
equatorward side and decrease 
more slowly on the poleward side. 
Density cavity observed on steep 
temperature gradient.
Heating Rate (black) =  W⊥
•
(eV) = q 2mψ
perp sc  V
perp
https://ntrs.nasa.gov/search.jsp?R=20090001282 2019-08-30T05:52:22+00:00Z


On Alfvenic Waves and Stochastic Ion Heating with 1Re Observations of Strong Field-aligned Currents, Electric Fields, and O+ ions

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