Yiding Chen

Does the F10.7 index correctly describe solar EUV flux during the deep solar minimum of 2007–2009?

This paper shows that the relationship between solar EUV flux and the F-10.7 index during the extended solar minimum (Smin) of 2007-2009 is different from that in the previous Smin. This difference is also seen in the relationship between f(o)F(2) and F-10.7. We collected SOHO/SEM EUV observations and the F-10.7 index, through June 2010, to investigate solar irradiance in the recent Smin. We find that, owing to F-10.7 and solar EUV flux decreased from the last Smin to the recent one with different amplitudes (larger in EUV flux), EUV flux is significantly lower in the recent Smin than in the last one for the same F-10.7. Namely, F-10.7 does not describe solar EUV irradiance in the recent Smin as it did in the last Smin. That caused remarkable responses in ionospheric f(o)F(2). For the same F-10.7, f(o)F(2) in the recent Smin is lower than that in the last one; further, it is also lower than that in other previous Smins. Therefore, F-10.7 is not an ideal indicator of f(o)F(2) during the recent Smin, which implies that F-10.7 is not an ideal proxy for solar EUV irradiance during this period, although it has been adequate during previous Smins. Solar irradiance models and ionospheric models will need to take this into account for solar cycle investigations.

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.

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.

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.

The long-duration positive storm effects in the equatorial ionosphere over Jicamarca

The long-duration positive storm (LPS) in the equatorial regions is relatively poorly understood. In this report, we conducted a statistical analysis of the LPS effects in the equatorial ionosphere over Jicamarca (12.0°S, 283.2°E) in 1998–2010. There are 250 geomagnetic storms (minimum Dst < −50 nT) in 1998–2010, but the ionosonde observations at Jicamarca are available only for 204 storms. A total of 46 LPSs are identified in terms of the criterion that the storm time relative deviation of peak density of F2 layer (NmF2) exceeds 25% for more than 6 h. A salient feature is that the occurrence of LPSs tends to decay approximately exponentially on the following days after the main phase of geomagnetic storms. The ratios of the number of equatorial LPSs to that of geomagnetic storms have no obvious dependence on season and solar activity. During the daytime LPSs, the disturbed zonal electric field is mostly westward, as indicated from the geomagnetic field changes in the equatorial American region. For the nighttime LPSs, the significant uplifting of F2 layer caused by an eastward electric field is the most important feature. Therefore, the disturbed electric field should play an essential role in forming the equatorial LPSs.

Modeling study of nighttime enhancements in F region electron density at low latitudes

In this paper we carry out case-control simulations to examine the mechanisms of the nighttime anomalous enhancements in electron density in the ionosphere at low latitudes, which were studied earlier in another research. The results confirm the earlier conclusion that the downward E × B plasma drift due to westward electric field at night is the main driving force for the nighttime enhancement. In addition, the phase of the electric field is found important in forming the enhancement. Delayed westward electric field can produce significant postmidnight enhancement as observed at Sanya (geomagnetic latitude: 8.2°N). In addition, the equatorward neutral wind at night is found to modulate the formation of the nighttime enhancement at geomagnetic latitudes below 15°N. The combined effects of the two drivers cause significant equatorward/downward plasma flux, which results in the enhancement of electron density as well as the drop in ionospheric peak height.

How does ionospheric TEC vary if solar EUV irradiance continuously decreases

It is an interesting topic how the ionosphere varies when solar extreme ultraviolet (EUV) irradiance decreases far below normal levels. When extrapolating the total electron content (TEC)-EUV relation, significantly negative TECs at the zero solar EUV point are obtained, which indicates that TEC-EUV variation under extremely low solar EUV (ELSE) conditions does not follow the TEC-EUV trend during normal solar cycles. We suggest that there are four types of nonlinear TEC-EUV variations over the whole EUV range from zero to the solar maximum level. The features of the ionosphere under ELSE conditions were investigated using the TEC extrapolated with cubic TEC-EUV fitting. With the constraint of zero TEC at zero EUV, the cubic fitting takes not only observations but also the trend of the ionosphere (only an extremely weak ionosphere can exist when EUV vanishes) into account. The climatology features of TEC under ELSE conditions may differ from those during normal solar cycles at nighttime. Ionospheric dynamic processes are supposed to still significantly affect the ionosphere under ELSE conditions and induce this difference. With solar EUV decreasing, global electron content (GEC) should vary largely in accordance with the GEC-EUV trend during normal solar cycles, and the seasonal fluctuation of GEC declines, owing to the contraction of the ionosphere.

NmF2 enhancement during ionospheric F2 region nighttime: A statistical analysis based on COSMIC observations during the 2007–2009 solar minimum

In this paper the global features of NmF2 enhancement occurring during ionospheric F2 region nighttime (the period when the sunlight is occulted by the Earth in the altitudinal range of ionospheric F2 region) and lasting for more than 2 h were investigated based on Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) measurements during the 2007–2009 solar minimum. This nighttime enhancement of NmF2 mainly appears at the latitudes with dips larger than 45° in the winter hemisphere in solstice seasons. The magnitude of NmF2 enhancement reaches latitudinal maxima (minima) at the geomagnetic latitudes of about 40°–50° (60°–70°), with larger magnitudes in the northern winter hemisphere than in the southern winter hemisphere. The longitudinal variation of nighttime enhancement is also evident; especially the magnitude of NmF2 enhancement shows a significant longitudinal modulation in the southern winter hemisphere. The controlling factors of the spatial variations of NmF2 nighttime enhancement were analyzed. The longitudinal variation of NmF2 nighttime enhancement is suggested to be related to the longitudinal differences in background NmF2, thermospheric density, and interhemispheric plasma transport, and the latitudinal variation of NmF2 nighttime enhancement is possibly related to the latitudinal variations of geomagnetic inclination and the plasma storage in the topside ionosphere and the plasmasphere. The configuration of the geomagnetic field plays an important role in the longitudinal and latitudinal variations of NmF2 nighttime enhancement.

Geomagnetic activity effect on the global ionosphere during the 2007–2009 deep solar minimum

In this paper the significant effect of weaker geomagnetic activity during the 2007–2009 deep solar minimum on ionospheric variability on the shorter-term time scales of several days was highlighted via investigating the response of daily mean global electron content (GEC, the global area integral of total electron content derived from ground-based GPS measurements) to geomagnetic activity index Ap. Based on a case during the deep solar minimum, the effect of the recurrent weaker geomagnetic disturbances on the ionosphere was evident. Statistical analyses indicate that the effect of weaker geomagnetic activity on GEC variations on shorter-term time scales was significant during 2007–2009 even under relatively quiet geomagnetic activity condition; daily mean GEC was positively correlated with geomagnetic activity. However, GEC variations on shorter-term time scales were poorly correlated with geomagnetic activity during the solar cycle descending phase of 2003–2005 except under strong geomagnetic disturbance condition. Statistically, the effects of solar EUV irradiance, geomagnetic activity, and other factors (e.g., meteorological sources) on GEC variations on shorter-term time scales were basically equivalent during the 2007–2009 solar minimum.

Deriving the effective scale height in the topside ionosphere based on ionosonde and satellite in situ observations

Chapman scale height is a valuable key parameter measuring the shape of the profile of plasma density in the F2 layer ionosphere. Currently, the data of Chapman scale height are routinely derived from ionogram observations at many ionosonde stations in terms of the SAO explorer software. In this report, we collected the in situ observations of plasma density at altitudes around 600 km from the ROCSAT-1 satellite and of simultaneous F peak parameters from an ionosonde operated at Wuhan (30.6°N, 114.4°E), a low-latitude station in central China, to estimate the topside plasma density profiles by using the Chapman α function and further retrieve Chapman scale height. Evident solar cycle, seasonal variation, and local time variation are presented in the retrieved Chapman scale height over Wuhan. The climatological features of the derived Chapman scale height are significantly different from those from the ground-based ionograms. Such significant discrepancy suggests that further improvements are required in the present extrapolating topside electron density profiles from ionosonde observations. Furthermore, the attempt to constructing plasma density profiles through combining ionosonde and satellite in situ observations provides a new way to reanalyze observations from different sources and normalize plasma density recorded at varying altitudes to specified altitudes, which is critical and more convenient for ionospheric climatology studies.