Statistical Study of the Non-thermal Continuum Radiation Beaming Angle
measured by the High Frequency Receiver on Van Allen Probes-A
Abstract
The nonthermal continuum radiation (NTC) beaming angle is computed over
the entire Van Allen Probes A mission when the spacecraft was in the
dawn sector. The conditions in the dawn sector are favorable for the
wave vector to lie near/in the spacecraft’s spin plan allowing a
favorable estimate of the beaming angle, and the dawn sector is also
advantageous in that previous studies show NTC occurrence to peak in
this sector. We found that scatter plots, over the entire mission, of
beaming angle versus magnetic latitude form a distinct inverted V
pattern, with the apex at/near the magnetic equator. This pattern was
sharpest for frequencies (f) ≲ 100 kHz. Using the NTC beaming formula
from LMCT, we show that such an inverted V pattern is expected due to
the large variation in the plasmapause location over the entire mission.
The theoretical derived pattern qualitatively reproduces the observed
pattern but not quantitatively. The lack of quantitative agreement is
discussed and is attributed to several factors, one factor is off
centered emissions from the radio window. The qualitative agreement
strongly supports LMCT as being the dominant mechanism generating NTC
for f ≲ 100 kHz. For f ≳ 100 kHz the inverted V pattern becomes less
distinct, and strong near equatorial beaming is observed. After
considering contamination of our selections by left-handed polarized
AKR, our study suggests that besides LMCT another unidentified NTC
generation mechanism becomes important for f ≳ 100 kHz.