A. Howarth

Low‐Altitude Ion Heating, Downflowing Ions, and BBELF Waves in the Return Current Region

Heavy (O+) ion energization and field-aligned motion in and near the ionosphere are still not well understood. Based on observations from the CASSIOPE Enhanced Polar Outflow Probe (e-POP) at altitudes between 325 km and 730 km over one year, we present a statistical study (24 events) of ion heating and its relation to field-aligned ion bulk flow velocity, low-frequency waves and field-aligned currents (FACs). The ion temperature and field-aligned bulk flow velocity are derived from 2-D ion velocity distribution functions measured by the suprathermal electron imager (SEI) instrument. Consistent ion heating and flow velocity characteristics are observed from both the SEI and the rapid-scanning ion mass spectrometer (IRM) instruments. We find that transverse O+ ion heating in the ionosphere can be intense (up to 4.5 eV), confined to very narrow regions (∼ 2 km across B), is more likely to occur in the downward current region, and is associated with broadband extremely low frequency (BBELF) waves. These waves are interpreted as linearly polarized perpendicular to the magnetic field. The amount of ion heating cannot be explained by frictional heating, and the correlation of ion heating with BBELF waves suggest that significant wave-ion heating is occurring and even dominating at altitudes as low as 350 km, a boundary that is lower than previously reported. Surprisingly, the majority of these heating events (17 out 24) are associated with core ion downflows rather than upflows. This may be explained by a downward-pointing electric field in the low-altitude return current region.

Enhanced Polar Outflow Probe Ionospheric Radio Occultation Measurements at High Latitudes: Receiver Bias Estimation and Comparison With Ground‐Based Observations

This paper presents validation of ionospheric Global Positioning System (GPS) radio occultation measurements of the GPS Attitude, Positioning, and Profiling Experiment occultation receiver (GAP-O). GAP is one of eight instruments comprising the Enhanced Polar Outflow Probe (e-POP) instrument suite on board the Cascade Smallsat and Ionospheric Polar Explorer (CASSIOPE) satellite. One of the main error sources for certain GAP-O data products is the receiver differential code bias (rDCB). A minimization of standard deviations (MSD) technique has shown the most promise for rDCB estimation, with estimates ranging primarily from −40 to −28 total electron content units (TECU = 1016xa0elxa0m−2; 21.6 to 15.1xa0ns), including a long-term decrease in rDCB magnitude and variability over the first 3xa0years of instrument operation. In application of the MSD method, the sensitivity of bias estimates to ionospheric shell height are as large as 4.5xa0TECU per 100xa0km. MSD calculations also agree well with the “assumption of zero topside TEC” method for rDCB estimate at satellite apogee. Bias-corrected topside TEC of GAP-O was validated by statistical comparison with topside TEC obtained from ground-based GPS TEC and ionosonde measurements. Although GAP-O and ground-based topside TEC had similar variability, GAP-O consistently underestimated the ground-derived topside TEC by up to 7xa0TECU. Ionospheric electron density profiles obtained from Abel inversion of GAP-O occultation TEC showed good agreement with F region densities of ground-based incoherent scatter radar measurements. Comparison of GAP-O and ionosonde measurements revealed correlation coefficients of 0.78 and 0.79, for peak F region density and altitude, respectively.

Citizen Radio Science: An Analysis of Amateur Radio Transmissions With e‐POP RRI

We report the results of a radio science experiment involving citizen scientists conducted on 28 June 2015, in which the Radio Receiver Instrument (RRI) on the Enhanced Polar Outflow Probe (e-POP) tuned-in to the 40 and 80 m Ham Radio bands during the 2015 American Radio Relay League (ARRL) Field Day. We have aurally decoded the Morse coded call signs of 14 Hams (amateur operators) from RRI’s data to help ascertain their locations during the experiment. Through careful analysis of the Hams’ transmissions, and with the aid of ray tracing tools, we have identified two notable magnetoionic effects in the received signals: plasma cutoff and single-mode fading. The signature of the former effect appeared approximately 30 seconds into the experiment, with the sudden cessation of signals received by RRI despite measurements from a network of ground-based receivers showing that the Hams’ transmissions were unabated throughout the experiment. The latter effect, single-mode fading, was detected as a double-peak modulation on the individual “dots” and “dashes” of one the Ham’s Morse coded transmissions. We show that the modulation in the Ham’s signal agrees with expected fading rate for singlemode fading. The results of this experiment demonstrate that Ham Radio transmissions are a valuable tool for studying radio wave propagation and remotely sensing the ionosphere. The analysis and results provide a basis for future collaborations in radio science between traditional researchers in academia and industry, and citizen scientists in which novel and compelling experiments can be performed. Plain Language Summary We report the results of an experiment in which we used a satellite-based radio receiver to eavesdrop on Ham radio communications as the satellite passed over the United States. We identified 14 Ham radio users by their call signs, and used this information to determine their location during the experiment. We were able to identify unique signatures in the Hams’ signals that are directly related to the nature of the how the Hams’ radio waves traveled through the Earth’s ionosphere up to the satellite. Furthermore, we used our knowledge of the position of the spacecraft, and the location of the Hams and their broadcast frequencies to deduce the structure of the Earth’s ionosphere over the United States during the experiment. This experiment and its results show that Ham radio transmissions and Hams (amateur radio operators) can be valuable assets in determining the structure of the ionosphere over large geographic regions.

Receiver bias estimation and validation of e-POP GAP-O ionospheric radio occultation measurements

This paper presents validation of ionospheric Global Positioning System (GPS) radio occultation (RO) measurements of the GPS Attitude, Positioning, and Profiling Experiment occultation receiver (GAP-O). The primary source of uncertainty impacting GAP-O data products is the receiver differential code bias (rDCB). A minimization of standard deviations (MSD) technique for rDCB estimate has shown the most promise, and resulted in estimates ranging from −39 to −29 TECU, including a steady, long term decrease in rDCB magnitude. MSD estimates agree well with the “assumption of zero topside TEC” method at satellite apogee in the polar cap. Bias-corrected topside TEC of GAP-O was validated by statistical comparison with topside TEC obtained from ground-based GPS TEC and ionosonde measurements. GAP-O and ground-based topside TEC had similar variability, however GAP-O consistently underestimated the ground-derived topside TEC by up to 8 TECU. Ionospheric electron density profiles obtained from Abel inversion of GAP-O occultation TEC showed consistently good agreement with F-region densities of incoherent scatter radar measurements, however RO-derived E-region densities were not as reliable at high latitudes.

First satellite imaging of auroral pulsations by the Fast Auroral Imager on e-POP

We report the first satellite imaging of auroral pulsations by the Fast Auroral Imager (FAI) on board the Enhanced Polar Outflow Probe (e-POP) satellite. The near-infrared camera of FAI is capable of providing up to two auroral images per second, ideal for investigation of pulsating auroras. The auroral pulsations were observed within the auroral bulge formed during a substorm interval on 19 February 2014. This first satellite view of these pulsations from FAI reveals that (1) several pulsating auroral channels (PACs) occur within the auroral bulge, (2) periods of the intensity pulsations span over one decade within the auroral bulge, and (3) there is no apparent trend of longer pulsation periods associated with higher latitudes for these PACs. Although PACs resemble in some respect stable pulsating auroras reported previously, they have several important differences in characteristics.