Receivers located at Two Rivers, Alaska (64.9øN, 146.9øW), and Circle Hot Springs, Alaska (65.5øN, 144.7øW), have been used to monitor the spectrum between 0.05-4.8 MHz for extended periods. Seasonal and diurnal effects of auroral roar, a weak narrow band radio emission near 2 and 3 times the ionospheric electron cyclotron frequency (f•), have been determined. Many individual auroral roar events correlate with magnetic activity, and superposed epoch analysis using planetary K indices shows a correlation between magnetic activity and auroral roar commencement. For a 3f• roar event on March 23, 1992, riometer, magnetometer, photometer, and all-sky camera data are available and show that individual bursts of 3f• auroral roar are associated with intensifications of the aurora, as was known previously for 2f• roar. Finally, two possible generation mechanisms are investigated in detail: nonlinear interaction between lower hybrid and electrostatic upper hybrid waves producing electromagnetic waves, and the feasibility of a lower ionospheric decameter maser for direct generation of X mode waves at harmonics of f•. Introduction Although reports of natural radio emissions from aurora have appeared occasionally during the past four (s• •vi•ws by Ellyett [1969], LaBelk [1989], and LaBelk and Weatherwax [1992]), only in the past few years has a systematic study been done covering the 0.05-to 4.8-MHz frequency range for extended time periods. Surprisingly, this rather ordinary frequency range, which includes the familiar AM broadcast band, exhibits phenomena of auroral origin which are neither well described observationally nor explained theoretically. The study reported here uses passive groundbased receivers at remote high-latitude regions in the northern hemisphere where interesting and still unexplained radio events have been reported in previous papers. These phenomena include broadband noise enhancements [Weatherwax et al., 1994a], bursty broadband emissions at 1.4-3.5 MHz [Weatherwax et al., 1994b], and narrow-band emissions near 2 [Kellogg and Monson, 1979; 1984] and 3 times [Weatherwax et al., 1993] the ionospheric electron cyclotron frequency (fc,). Emissions near 2fc• were first observed at Churchill, Manitoba (latitude 58.8øN, longitude 94.0øW, magnetic latitude 69.7øN), by Kellogg and Monson [1979], who labeled them auroral "roar." These emissions occurred at frequencies between 2.9 and 3.1 MHz and lasted up to tens of minutes. Kellogg and Monson [1984] observed 69 events during several campaigns over three winters at Churchill. The most commonly observed frequency of 3.03 MHz corresponds to 2f• at an altitude of 275 km above Churchill. The events were usually associated with auroral breakup or with westward traveling surges. Attempts to observe similar events from Cleary, Alaska (latitude 65.1øN, longitude 147.8øW, magnetic latitude 64.8øN), during a 2-week campaign in 1982, were unsuccessful. Recent observations of 2f• auroral roar near Fairbanks, Alaska, were reported by Weatherwax et al. [1993]. These emission...

Poleward moving transients have been proposed to be ionospheric signatures of plasma transfer events taking place at the dayside magnetopause. They are usually observed to brighten at the equatorward edge of the dayside auroral oval and fade as they move into the polar cap. This paper reports the observation of a new type of poleward moving dayside auroral transient which has several cycles of intensity variations. Observations of these transients show a series of intensifications in brightness along the arc or rayed band during poleward motion accompanied by a brightening in the auroral oval. As they reach their extreme poleward position they brighten and than fade from view. This brightening sequence may be explained by multiple reconnection of the magnetic flux tube associated with the transient.

The Community Coordinated Modeling Center has been leading community‐wide space science and space weather model validation projects for many years. These efforts have been broadened and extended via the newly launched International Forum for Space Weather Modeling Capabilities Assessment (https://ccmc.gsfc.nasa.gov/assessment/). Its objective is to track space weather models' progress and performance over time, a capability that is critically needed in space weather operations and different user communities in general. The Space Radiation and Plasma Effects Working Team of the aforementioned International Forum works on one of the many focused evaluation topics and deals with five different subtopics (https://ccmc.gsfc.nasa.gov/assessment/topics/radiation-all.php) and varieties of particle populations: Surface Charging from tens of eV to 50‐keV electrons and internal charging due to energetic electrons from hundreds keV to several MeVs. Single‐event effects from solar energetic particles and galactic cosmic rays (several MeV to TeV), total dose due to accumulation of doses from electrons (>100 keV) and protons (>1 MeV) in a broad energy range, and radiation effects from solar energetic particles and galactic cosmic rays at aviation altitudes. A unique aspect of the Space Radiation and Plasma Effects focus area is that it bridges the space environments, engineering, and user communities. The intent of the paper is to provide an overview of the current status and to suggest a guide for how to best validate space environment models for operational/engineering use, which includes selection of essential space environment and effect quantities and appropriate metrics.

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The SCIFER sounding rocket was launched over the dayside aurora, at 10 hr Magnetic Local Time (MLT) on January 25, 1995. Meridian‐scanning photometers (MSP) and all‐sky television (ASTV) systems were operated at Longyearbyen (LYR) and Ny‐Ålesund (NYA) on Svalbard under the flight apogee to facilitate the launch decision and identify the ionospheric signatures of the various energetic particle populations observed at the rocket. The characteristics of the 0.3 eV–15.5 KeV electron populations producing the observed aurora were identified from the time, energy and pitch angle electron spectrometer data throughout the SCIFER flight. The optical data clearly showed the location of the trapping boundary seen by the SCIFER energetic electron spectra. Equatorward of the boundary, pulsating patches of auroral luminosity corresponded in pulsation period and location to the >4 keV energy‐dispersed electrons observed at SCIFER. The 10 eV electron flux rather uniformly distributed equatorward from the trapping boundary is accounted for by photoelectrons from the conjugate region, producing most of the 6300 Å [OI] emission observed south of the zenith in the MSP data. Poleward of the trapping boundary, relatively bright, discrete arcs and bands typical of the dayside auroral oval were observed. These corresponded to the inverted V and field‐aligned electron populations which were observed at SCIFER to be < 1 keV characteristic energy and to range from 2 to 5 ergs cm−2 sec−1. The electron populations and the resulting arcs were remarkably similar to those observed on the high latitude nightside (poleward of the trapping boundary) but with obviously different source region. There was no persistent 6300 Å [OI] auroral emission observed poleward of the trapping boundary consistent with the particle observations. Hydrogen emissions were associated with the discrete aurora, indicating that the main proton energy flux was poleward of the trapping boundary.

The rapid increase of electron temperature in the early morning hours at low latitudes is a well‐known ionospheric phenomenon called morning overshoot. In this study, we extensively investigate the dependence of morning overshoot on local time, season, latitude/longitude/altitude, and magnetic activity. The electron temperature and density data set used in this study are obtained from (1) the Swarm constellation at two different altitudes of 470 and 520 km with identical payloads and (2) the Floating Potential Measurement Unit onboard International Space Station at an altitude of 400 km. Based on the data between 2014 and 2019, the main findings of this study are as follows: (1) on a global average, morning overshoot generally weakens with decreasing altitudes. (2) Morning overshoot is stronger around the dip equator than at midlatitude regions. As latitude increases, the overshoot decreases gradually and shifts to later local times. (3) In off‐equatorial regions the overshoot is stronger in the winter than in the summer hemisphere, especially at higher altitudes. (4) Lastly, the morning overshoot shows multiday oscillations, which are negatively correlated with plasma density and affected by geomagnetic activity.

The CCDs on the Chandra X-ray Observatory are vulnerable to radiation damage from low-energy protons scattered off

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