Huijun Le

A study of the Weddell Sea Anomaly observed by FORMOSAT‐3/COSMIC

[1] More than two years of COSMIC electron density profiles at low solar activities are collected to study the evolution of the Weddell Sea Anomaly (WSA), which appears as an evening enhancement in electron density during local summer. Observations show that the change in NmF2 (the F2 peak electron density) is associated with the change in hmF2 (the F2 peak height), while the latter is correlated closely with the components of the geomagnetic field. We find that (1) in the afternoon, hmF2 is more liable to rise drastically in regions with a larger jsin(2I)j value, which would occur early at certain declinations, eastward in the southern hemisphere and westward in the northern hemisphere; (2) subsequently, a larger increment of hmF2 is coincidentally followed by a stronger enhancement of NmF2 and the enhancement ends just around the local sunset; and (3) in midlatitudes, the evolution pattern of hmF2 in the evening of equinoxes and winter is similar to that in summer, albeit without a lasting NmF2 enhancement as that in summer. These features suggest that the NmF2 enhancement and the hmF2 increase could arise from the thermospheric wind effect, and solar photoionization plays a crucial role in the enhancement as well. The general midlatitude F2 layer enhancement in local summer evening is consistent with the WSA on the above features, indicating that the WSA is a manifestation, with a particular geometry of the magnetic field, of the evening enhancement induced by the winds.

An analysis of the scale heights in the lower topside ionosphere based on the Arecibo incoherent scatter radar measurements

[1] We statistically analyze the ionospheric scale heights in the lower topside ionosphere based on the electron density (Ne) and temperature profiles observed from the incoherent scatter radar (ISR) at Arecibo (293.2E, 18.3N), Puerto Rico. In this study, a database containing the Arecibo ISR observations from 1966 to 2002 has been used in order to investigate the diurnal and seasonal variations and solar activity dependences of the vertical scale height (VSH), which is deduced from the electron concentration profiles

Features of the middle- and low-latitude ionosphere during solar minimum as revealed from COSMIC radio occultation measurements

In this study, the ionospheric electron density profiles retrieved from radio occultation measurements of the Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) mission are analyzed to determine the F-2 layer maximum electron density (NmF2), peak height (h(m)F(2)), and Chapman scale height (H-m). During the deep solar minimum of 2008-2009, NmF2, h(m)F(2), and H-m show complicated seasonal variations, which are generally consistent with those in previous solar minima. Besides the equinoctial asymmetry, nonseasonal and semiannual anomalies are present in daytime NmF2; the Weddell Sea anomaly appears in nighttime NmF2 in all seasons except the June solstice. Unusually higher values of h(m)F(2) and H-m appear at southern middle latitudes in the region centered at 70 degrees E in the daytime and h(m)F(2) at 70 degrees W in the nighttime. Wave-like longitudinal patterns are evidently present at low latitudes in all three parameters, showing diurnal and seasonal nature. The values of the parameters under study are smaller in 2008-2009 than the rest of the COSMIC period examined in this study. The seasonal and latitudinal pattern of daytime NmF2 on the solar sensitivity not only confirms our earlier investigation but also explains the observed small NmF2 in 2008-2009 in response to the reduced solar extreme ultraviolet radiance.

Latitudinal dependence of the ionospheric response to solar eclipses

[1] In this study, we statistically analyze the latitudinal dependence of F2-layer peak electron densities (NmF2) and total electron content (TEC) responses to solar eclipses by using the ionosonde observations during 15 eclipse events from 1973 to 2006 and the GPS TEC observations during six solar eclipse events from 1999 to 2006. We carried out a model study on the latitudinal dependence of eclipse effects on the ionosphere by running a theoretical ionospheric model with the total eclipse occurring at 13 latitudes from 0 Nt o 60N at intervals of 5. Both the observations and simulations show that the NmF2 and TEC responses have the same latitudinal dependence: the eclipse effects on NmF2 and TEC are smaller at low latitudes than at middle latitudes; at the middle latitudes (>40), the eclipse effect decreases with increasing latitude. The simulations show that the smaller NmF2 responses at low latitudes are mainly because of much higher heights of hmF2 at low latitudes and electron density response decreasing rapidly with increasing height. For the eclipse effects at the middle latitudes (>40), the simulations show that the smaller NmF2 or TEC response at higher latitude is mainly ascribed to the larger downward diffusion flux induced by the larger dip angle at this region, which can partly make up for the plasma loss and alleviate the depression of electron density in the F region. The simulated results show that there is an overall decrease in electron temperature throughout the entire height range at the middle latitude, but for the low latitudes the eclipse effect on electron temperature is much smaller at high heights, which is mainly because of the much smaller reduction of photoelectron production rate at its conjugate low heights where only a partial eclipse with small eclipse magnitude occurs.

The midlatitude F2 layer during solar eclipses: Observations and modeling

National Natural Science Foundation of China[40725014, 40674090, 40636032]; National Important Basic Research Project[2006CB806306]

Comparative study of the equatorial ionosphere over Jicamarca during recent two solar minima

It is a critical issue that whether or not the extremely deep solar minimum of solar cycle 23/24 brought serious influences on the Earths space environment. In this study, we collected and manually scaled the ionograms recorded by a DPS ionosonde at Jicamarca (12.0 degrees S, 283.2 degrees E) to retrieve F layer parameters and electron density (N-e) profiles. A comparative study is performed to evaluate the equatorial ionosphere in solar minima of cycle 22/23 (1996-1997) and 23/24 (2008-2009). The seasonal median values of the critical frequency of F-2 layer (f(o)F(2)) were remarkably reduced in four seasons during the deep solar minimum, compared to those in 1996-1997. It is the first time to find that lower values prevail at most times in 2008-2009 in the F2 layer peak height (h(m)F(2)) and Chapman scale height (H-m). The bottomside profile thickness (B0) shows higher values in 2008-2009 than that in 1996-1997 at some daytime intervals, although it also becomes smaller during the rest times. Furthermore, the ionosphere in 2008-2009 is contracted strongly at altitudes above h(m)F(2) and more perceptible in the afternoon hours. The decrease in N-e is strongest in September equinox and weakest in June solstice. The ionospheric responses from solar minimum to minimum are mainly caused by the reduction in solar extreme ultraviolet intensity, and the contribution from dynamical processes competes and is variable. Analysis reveals that semiannual and longer-scale components are certainly reduced during the deep solar minimum, while shorter scale (e. g., 4 month) components may disrupt the decline picture at some times.

Features of the F3 layer in the low‐latitude ionosphere at sunset

The F-3 layer is a common feature within +/- 10 degrees of the magnetic equatorial ionosphere in the daytime. According to Balan et al. (1998) the F-3 layer occurs mainly during the morning-noon period due to the combined effect of the upward E x B drift and the neutral wind that provides upward plasma drifts at and above the F-2 layer. The F-3 layer occurrence rate is higher in summer and decreases with increasing solar activity. In this study, the characteristic of the sunset F-3 layer is first investigated using a solar cycle of ionosonde data (1995-2010) from the magnetic equatorial station at Jicamarca, and compared with the features derived from the four subtropical stations at Sao Luis, Fortaleza, Kwajalein, and Vanimo. Evidence shows that the local time distribution of the occurrence of the F-3 layer can extend to the postsunset time (1800-2100 local time). The sunset F-3 layer has a strong seasonal dependence occurring mainly during the summertime. Unlike the daytime F-3 layer, the occurrence of the sunset F-3 layer clearly increases and the virtual height of the bottom side of the F-3 layer statistically increases from 620 to 1000 km with increasing solar activity. In addition, the occurrence of the sunset F-3 layer at the other stations is much less than that at Jicamarca. These features of the dependence on the season, solar activity, and latitude are clearly related to the geomagnetic control of the evening prereversal enhancement of the equatorial zonal electric field and geomagnetic configuration.

An analysis of thermospheric density response to solar flares during 2001-2006

Previous studies show there are significant thermospheric responses to the two great solar flares on October 28, 2003 (X17.2) and November 4, 2003 (X28). In the present study, we further explored the thermospheric response to all X-class solar flares during 2001-2006. The observed results show that X5 and stronger solar flares can induce an average enhancement of 10-13% in thermospheric density in latitude 50 degrees S-50 degrees N within similar to 4 h after the flare onset. Many important lines and continua in solar EUV region are optically thick, thus EUV enhancements are smaller for flares located near the solar limb due to absorption by the solar atmosphere. Limb flares induce smaller thermospheric responses, due to the limb effect of solar EUV. The thermospheric density enhancement is much more correlated with integrated EUV flux than with peak EUV flux, with a high correlation coefficient of 0.91, which suggests that thermospheric response is strongly dependent on the total integrated energy into the thermosphere.

Statistical analysis of solar EUV and X-ray flux enhancements induced by solar flares and its implication to upper atmosphere

The 0.1-0.8 nm X-ray flux data and 26-34 nm EUV flux data are used to statistically analyze the relationship between enhancement in X-ray flux and that in EUV flux during solar flares in 1996-2006. The EUV enhancement does not linearly increase with X-ray flux from C-class to X-class flares. Its uprising amplitude decreases with X-ray flux. The correlation coefficients between enhancements in EUV and X-ray flux for X, M and C-class flares are only 0.66, 0.58 and 0.54, respectively, which suggests that X-ray flux is not a good index for EUV flux during solar flares. Thus, for studying more accurately solar flare effect on the ionosphere/thermosphere system, one needs to use directly EUV flux measurements. One of important reasons for depressing relationship between X-ray and EUV is that the central meridian distance (CMD) of flare location can significantly affect EUV flux variation particularly for X-class flares: the larger value of CMD results in the smaller EUV enhancement. However, there are much smaller CMD effects on EUV enhancement for M and C-class flares. The solar disc images from SOHO/EIT are utilized to estimate the percentage contribution to total EUV enhancement from the flare region and from other region. The results show the larger percentage contribution from other region for the weaker flares, which would reduce the loss of EUV radiation due to limb location of flare and then weaken the CMD effect for weaker flares like M and C-class.

Effects of disturbed electric fields in the low‐latitude and equatorial ionosphere during the 2015 St. Patrick's Day storm

The 2015 St. Patricks day geomagnetic storm with SYM-H value of -233 nT is an extreme space weather event in the current 24th solar cycle. In this work, we investigated the main mechanisms of the profound ionospheric disturbances over equatorial and low latitudes in the Asian-Australian sector and the American sector during this super storm event. The results reveal that the disturbed electric fields, which comprise penetration electric fields (PEFs) and disturbance dynamo electric fields (DDEFs), play a decisive role in the ionospheric storm effects in low latitude and equatorial regions. PEFs occur on March 17 in both the American sector and the Asian-Australian sector. The effects of DDEFs are also remarkable in the two longitudinal sectors. Both the DDEFs and PEFs show the notable local time dependence, which causes the sector differences in the characteristics of the disturbed electric fields. This differences would further lead to the sector differences in the low-latitude ionospheric response during this storm. The negative storm effects caused by the long-duration DDEFs are intense over the Asian-Australian sector, while the repeated elevations of hmF2 and the EIA intensifications caused by the multiple strong PEFs are more distinctive over the American sector. Especially, the storm time F3-layer features are caught on March 17 in the American equatorial region, proving the effects of the multiple strong eastward PEFs.