L.-C. Tsai

Peculiar features of the low-latitude and midlatitude ionospheric response to the St. Patrick's Day geomagnetic storm of 17 March 2015

The current study aims at investigating and identifying the ionospheric effects of the geomagnetic storm that occurred during 17–19 March 2015. Incidentally, with SYM-H hitting a minimum of −232xa0nT, this was the strongest storm of the current solar cycle 24. The study investigates how the storm has affected the equatorial, low-latitude, and midlatitude ionosphere in the American and the European sectors using available ground-based ionosonde and GPS TEC (total electron content) data. The possible effects of prompt electric field penetration is observed in both sectors during the main phase of the storm. In the American sector, the coexistence of both positive and negative ionospheric storm phases are observed at low latitudes and midlatitudes to high latitudes, respectively. The positive storm phase is mainly due to the prompt penetration electric fields. The negative storm phase in the midlatitude region is a combined effect of disturbance dynamo electric fields, the equatorward shift of the midlatitude density trough, and the equatorward compression of the plasmapause in combination with chemical compositional changes. Strong negative ionospheric storm phase is observed in both ionosonde and TEC observations during the recovery phase which also shows a strong hemispherical asymmetry. Additionally, the variation of equatorial ionization anomaly as seen through the SWARM constellation plasma measurements across different longitudes has been discussed. We, also, take a look at the performance of the IRI Real-Time Assimilative Mapping during this storm as an ionospheric space weather tool.

Elliptical model of cutoff boundaries for the solar energetic particles measured by POES satellites in December 2006

[1]xa0Experimental data from a constellation of five NOAA Polar Orbiting Environmental Satellites (POES), satellites were used for studying the penetration of solar energetic particles (SEP) to high latitudes during long-lasting SEP events on 5–15 December 2006. We determined cutoff latitudes for electrons with energies >100 keV and >300 keV and for protons with energies from 240 keV to >140 MeV. The large number of satellites allowed us to derive snapshots of the cutoff boundaries with 1-hour time resolution. The boundaries were fitted well by ellipses. On the basis of the elliptical approach, we developed a model of cutoff latitudes for protons and electrons in the northern and southern hemispheres. The cutoff latitude is represented as a function of rigidity, R, of particles; MLT, geomagnetic indices Dst, Kp, and AE; and dipole tilt angle PS. The model predicts tailward and duskward shifting of the cutoff boundaries in relation to intensification of the cross-tail current, field-aligned currents, and symmetrical and asymmetrical parts of the ring current. The model was applied for prediction of polar cap absorption (PCA) effects observed at high latitudes by the Canadian Advanced Digital Ionosonde network of ionosondes. It was found that the PCA effects are related mainly to intense fluxes of >2.5 MeV protons and >100 keV electrons, which contribute mostly to the ionization of ionospheric D-layer at altitudes of ∼75 to 85 km. This finding was confirmed independently by FORMOSAT-3/COSMIC observations of the SEP-associated enhancements of electron content at altitudes of ∼80 km.

TEC evidence for near‐equatorial energy deposition by 30 keV electrons in the topside ionosphere

[1]xa0Observations of energetic electrons (10 – 300u2009keV) by NOAA/POES and DMSP satellites at heights <1000u2009km during the period from 1999 to 2010 allowed finding abnormal intense fluxes of ~106 – 107u2009cm−2u2009s−1u2009sr−1 for quasi-trapped electrons appearing within the forbidden zone of low latitudes over the African, Indo-China, and Pacific regions. Extreme fluxes appeared often in the early morning and persisted for several hours during the maximum and recovery phase of geomagnetic storms. We analyzed nine storm time events when extreme electron fluxes first appeared in the Eastern Hemisphere, then drifted further eastward toward the South-Atlantic Anomaly. Using the electron spectra, we estimated the possible ionization effect produced by quasi-trapped electrons in the topside ionosphere. The estimated ionization was found to be large enough to satisfy observed storm time increases in the ionospheric total electron content (TEC) determined for the same spatial and temporal ranges from global ionospheric maps. Additionally, extreme fluxes of quasi-trapped electrons were accompanied by the significant elevation of the low-latitude F-layer obtained from COSMIC/FORMOSAT-3 radio occultation measurements. We suggest that the storm time ExB drift of energetic electrons from the inner radiation belt is an important driver of positive ionospheric storms within low-latitude and equatorial regions.

Low-latitude ionospheric effects of energetic electrons during a recurrent magnetic storm

We study a magnetosphere-ionosphere coupling at low latitudes during a moderate (corotating interaction regions/high-speed solar wind streams-driven) geomagnetic storm on 22 July 2009. Recently, it has been shown that during major (coronal mass ejection-driven) storms, quasi-trapped >30u2009keV electrons largely enhance below the radiation belt in the forbidden zone and produce an additional ionization in the topside ionosphere. In this work, we examine a case of the recurrent storm when the magnetosphere-ionosphere coupling through the quasi-trapped electrons also may take place. Data from NOAA/Polar-orbiting Operational Environmental Satellite and Japanese Greenhouse gases Observing Satellite were used to identify the forbidden electron enhancement (FEE). We find a positive vertical gradient of the electron fluxes that indicates to the radiation belt as a source of FEE. Using global ionospheric maps, radiotomography reconstructions from beacon data and COSMIC/FORMOSAT-3 radio occultation measurements, we have observed an unusually large area in the nighttime ionosphere with increased total electron content (TEC) and prominent elevation of the F layer at low latitudes that coincides with FEEs spatially and temporarily. Ionizing particles are considered as an addition source of ionization along with generally accepted mechanisms for storm time TEC increase (a positive ionospheric storm). We discuss relative contributions of the FEE and disturbance dynamo electric field in the TEC increases during the storm recovery phase.

Longitudinal variations of positive dayside ionospheric storms related to recurrent geomagnetic storms

[1]xa0We have performed an analysis of case events and statistics of positive ionospheric storms in the dayside region of the equatorial ionization anomaly during recurrent geomagnetic storms (RGSs), which dominate in geomagnetic and ionospheric conditions on the declining phase of solar activity in 2004 to 2008. It is shown that total electron content (TEC) has a tendency to minimize before the beginning of RGSs and to peak 3 to 4u2009days after, i.e., on the RGS recovery phase produced by high-intensity long-duration continuous auroral activity. The maximum of TEC coincides with the maximum of solar wind velocity within high-speed solar wind streams. An analysis of electron content vertical profiles, derived from two independent methods using ionosondes and Constellation Observing System for Meteorology, Ionosphere, and Climate /Formosa Satellite mission-3 radio occultation, showed that in the maximum of an ionospheric storm on 28 March 2008, the F2 layer thickens, NmF2 increases by ~50%, and hmF2 elevates by a few tens of kilometers. The response of positive ionospheric storms to solar, heliospheric, and geomagnetic drivers reveals a prominent longitudinal asymmetry. In the longitudinal range from −90° to 90°, the solar illumination plays a major role, and in the range from 90° to −120°, the influence of heliospheric and geomagnetic drivers becomes significant. The highest correlations of the TEC enhancements with the heliospheric and geomagnetic drivers were found during December–February (r increased from ~0.3 to ~0.5). We speculate that the dynamics controlling this might result from an effect of solar zenith angle, storm time effects of thermospheric ΣO/N2 enhancement, and penetrating electric fields of interplanetary and magnetospheric origin.

Coseismic Traveling Ionospheric Disturbances during the Mw 7.8 Gorkha, Nepal, Earthquake on 25 April 2015 From Ground and Spaceborne Observations

Coseismic travelling ionospheric disturbances (CTIDs) and their propagation characteristics during Mw 7.8 Gorkha earthquake in Nepal on 25 April 2015 have been investigated using a suite of ground based GPS receivers and broad band seismometers along with the space borne Radio Occultation observations over the Indian subcontinent region. Depletion in vertical total electron content (VTEC), a so called ionospheric hole, is observed near the epicenter ~9 – 11 minutes after the onset of earthquake. A positive pulse preceding the depletion, similar to N-shaped perturbation, propagating with an apparent velocity of ~2.4 km/s is observed on the South. Further, the CTIDs in the southward direction are found to split in to fast (~2.4 – 1.7 km/s) and slow (~680 – 520 m/s) propagating modes at epicentral distances greater than ~800 km. However, the velocities of fast mode CTIDs are significantly smaller than the surface Rayleigh wave velocity (~3.7 km/s), indicating that they are not the true imprint of Rayleigh wave, instead, can probably be attributed to the superimposed wave front formed by the mixture of acoustic waves excited by main shock and propagating Rayleigh wave. The southward CTIDs are found to propagate at F2-region altitudes of ~300 – 440 km captured by COSMIC Radio Occultation observations. The CTIDs with periods of ~4 – 6 minutes are observed in all directions with significantly larger amplitudes and faster propagation velocities in South and East directions. The observed azimuthal asymmetry in the amplitudes and velocities of CTIDs are discussed in terms of the alignment with geomagnetic field and nature of surface crustal deformation during the earthquake.