Origin of the SuperDARN broad Doppler spectra:simultaneous observation with Oersted satellite magnetometer

We perform a case study of a favorable conjunction of an overpass of the Oersted satellite with the field-of-view of the SuperDARN Syowa East radar during an interval of the southward IMF <i>B<sub>z</sub></i>. At the time, the radar observed an L-shell aligned boundary in the spectral width around the dayside ionosphere. Simultaneously, high-frequency (0.2–5Hz) magnetic field fluctuations were observed by the Oersted satellite's high-time resolution magnetometer. These magnetic field fluctuations are considered to be Alfvén waves possibly associated with the particle which precipitates into the dayside high-latitude ionosphere when magnetic reconnection occurs. It has been theoretically predicted that the time-varying electric field is the dominant physical process to expand the broad HF radar Doppler spectra. Our observation clearly demonstrates that the boundary between narrow and broad spectral widths is corresponding well to the boundary in the level of the fluctuations, which supports the previous theoretical prediction. A close relationship between electric and magnetic field fluctuations and particle precipitations during southward IMF conditions has been confirmed by many authors. The present observation allows us to suggest that the boundary between narrow and broad Doppler spectral widths observed in the dayside ionosphere is connected with the signature of the open/closed field line boundary, such as the cusp particle precipitations via electric and magnetic field fluctuations for the case of the negative IMF <i>B<sub>z</sub></i> conditions.<br><br> <b>Key words.</b> Ionosphere (ionosphere-magnetosphere interactions; plasma convection). Magnetospheric physics (magnetopause, cusp, and boundary layers

Arctic and Antarctic polar mesosphere summer echoes observed with oblique incidence HF radars: analysis using simultaneous MF and VHF radar data

Polar mesosphere summer echoes (PMSEs) have been well studied using vertical incidence VHF radars at northern high-latitudes. In this paper, two PMSE events detected with the oblique incidence SuperDARN HF radars at Hankasalmi, Finland (62.3° N) and Syowa Station, Antarctica (69.0° S), are analyzed, together with simultaneous VHF and medium-frequency (MF) radar data. Altitude resolutions of the HF radars in the mesosphere and the lower thermosphere are too poor to know exact PMSE altitudes. However, a comparison of Doppler velocity from the HF radar and neutral wind velocity from the MF radar shows that PMSEs at the HF band appeared at altitudes within 80-90km, which are consistent with those from previous vertical incidence HF-VHF radar results. The HF-VHF PMSE occurrences exhibit a semidiurnal behavior, as observed by other researchers. It is found that in one event, PMSEs occurred when westward semidiurnal winds with large amplitude at 85-88km altitudes attained a maximum. When the HF-VHF PMSEs were observed at distances beyond 180km from MF radar sites, the MF radars detected no appreciable signatures of echo enhancement. <br><br><b>Key words.</b> Meteorology and atmospheric dynamics (middle atmosphere dynamics; thermospheric dynamics; waves and tides

Characteristics of medium-scale travelling ionospheric disturbances observed near the Antarctic Peninsula by HF radar

We present a survey of medium-scale traveling ionospheric disturbances (MSTIDs) observed by a Super Dual Auroral Radar Network HF radar located in the Falkland Islands between May 2010 and April 2011. The radar has a field of view that overlooks the Antarctic Peninsula, a known hot spot of gravity wave activity. We present observations of radar ground-backscatter data, in which the signatures of MSTIDs are manifested as structured enhancements in echo power. Observed periods were in the range 30–80 min, corresponding to frequencies of 0.2–0.6 mHz. Wavelengths were generally in the range 200–800 km and phase speeds in the range 100–300 m s−1. These values are within the ranges typically associated with medium-scale gravity waves. We find a primary population of northward (equatorward) propagating MSTIDs, which demonstrate an association with enhanced solar wind-magnetosphere coupling and a smaller, westward propagating population, that could be associated with atmospheric gravity waves excited by winds over the Andean and Antarctic Peninsula mountains or by the high winds of the Antarctic Polar Vortex

Comparison of flow angle variations of E-region echo characteristics at VHF and HF

In this study, characteristics of the auroral E-region echoes at two significantly different radar frequencies of 12 and 50 MHz are compared. Considered observations were performed at the Syowa Antarctic station in March of 1997 using two HF and one VHF radars at various angles with respect to the magnetic L shells. The diurnal variation of echo occurrence was found to be similar at two frequencies and consistent with previous studies. On the other hand, variation of echo occurrence with L-shell angle φ was shown to be significantly different at two frequencies. 50-MHz echoes were detected preferentially along the L shell (dominating direction of the electrojet flow) while 12-MHz echoes were detected in a broad range of azimuths with the maximum in echo occurrence at φ=40-50°. By plotting the Doppler velocity versus L-shell angle, we demonstrate that 12-MHz echoes can be divided into two populations, the high- and low-velocity echoes. The high-velocity echoes were observed mostly along the L shells while the low-velocity echoes were observed at all directions. We also show that the echo populations exhibit different variation of the Doppler velocity with the L-shell angle. We argue that while the 50-MHz echoes are related to the Farley-Buneman and gradient drift plasma instabilities, the 12-MHz echoes can have additional sources, such as the thermo-diffusion instability and/or neutral wind-related plasma instabilities

Simultaneous observations of the cusp with IMAGE Low Energy Neutral Atom Imager and SuperDARN radar

The Low Energy Neutral Atom (LENA) imager on the IMAGE spacecraft observed significant emission in the high-latitude magnetosheath direction during an extreme solar wind condition on April 11, 2001. The emission was modulated in such a manner that the sources shifted equatorward in the high-latitude sheath while sometimes undergoing brief poleward returns. This modulation and the IMF BZ tend to have correlative variations. During this interval of interest, SuperDARN was receiving strong backscattered signals from large potions of the dayside ionosphere. This observation indicates that the equatorward motion of the cusp latitude consists of rapid and slow phases. This kind of equatorward shift appears to correlate with the motion of the emission observed by LENA in the direction of the high-latitude sheath, which gives evidence for a means for monitoring the high-altitude cusp using IMAGE/LENA. It thus appears that the two remote sensing observations, i.e., IMAGE/LENA and SuperDARN radar would provide promising opportunities to understand the detailed dynamics of the polar cusp extending from the low-altitudes to the high-altitudes

Global diagnostics of ionospheric absorption during X-ray solar flares based on 8-20MHz noise measured by over-the-horizon radars

An analysis of noise attenuation during eighty solar flares between 2013 and 2017 was carried out at frequencies 8-20 MHz using thirty-four SuperDARN radars and the EKB ISTP SB RAS radar. The attenuation was determined on the basis of noise measurements performed by the radars during the intervals between transmitting periods. The location of the primary contributing ground sources of noise was found by consideration of the propagation paths of radar backscatter from the ground. The elevation angle for the ground echoes was determined through a new empirical model. It was used to determine the paths of the noise and the location of its source. The method was particularly well suited for daytime situations which had to be limited for the most part to only two crossings through the D region. Knowing the radio path was used to determine an equivalent vertical propagation attenuation factor. The change in the noise during solar flares was correlated with solar radiation lines measured by GOES/XRS, GOES/EUVS, SDO/AIA, SDO/EVE, SOHO/SEM and PROBA2/LYRA instruments. Radiation in the 1 to 8$\mathring{A}$ and and near 100$\mathring{A}$ are shown to be primarily responsible for the increase in the radionoise absorption, and by inference, for an increase in the D and E region density. The data are also shown to be consistent with a radar frequency dependence having a power law with an exponent of -1.6. This study shows that a new dataset can be made available to study D and E region.Comment: 30 pages, 5 figures, 2 tables. Submitted to Space Weathe

Direct comparison between magnetospheric plasma waves and polar mesosphere winter echoes in both hemispheres

We present the first and direct comparison between magnetospheric plasma waves and polar mesosphere winter echoes (PMWE) simultaneously observed by the conjugate observation with Arase satellite and high‐power atmospheric radars in both hemispheres, namely, the Program of the Antarctic Syowa Mesosphere, Stratosphere, and Troposphere/Incoherent Scatter Radar (PANSY) at Syowa Station (SYO; ‐69.00°S, 39.58°E), Antarctica, and the Middle Atmosphere Alomar Radar System (MAARSY) at Andøya (AND; 69.30°N, 16.04°E), Norway. The PMWE were observed during 03‐07 UT on March 21, 2017, just after the arrival of corotating interaction region (CIR) in front of high‐speed solar wind stream. An isolated substorm occurred at 04 UT during this interval. Electromagnetic ion cyclotron (EMIC) waves and whistler‐mode chorus waves were simultaneously observed near the magnetic equator and showed similar temporal variations to that of the PMWE. These results indicate that chorus waves as well as EMIC waves are drivers of precipitation of energetic electrons, including relativistic electrons, which make PMWE detectable at 55‐80 km altitude. Cosmic noise absorption (CNA) measured with a 38.2‐MHz imaging riometer and low‐altitude echoes at 55‐70 km measured with an MF radar at SYO also support the relativistic electron precipitation. We suggest a possible scenario in which the various phenomena observed in near‐Earth space, such as magnetospheric plasma waves (EMIC waves and chorus waves), pulsating auroras, CNA, and PMWE, can be explained by the interaction between the high‐speed solar wind containing CIRs and the magnetosphere