Equatorial sporadic E-layer abnormal density enhancement during the recovery phase of the December 2006 magnetic storm: A case study
© The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB. 2012
Received: 23 March 2011
Accepted: 18 October 2011
Published: 25 May 2012
Sporadic layers appear in the equatorial region (Esq) between 90 and 130 km mainly due to irregularities in the electrojet equatorial (EEJ) current. In the present work, we have analyzed the behavior of the frequency parameters associated with these sporadic layers, covering the days before, during, and subsequent to, the intense magnetic storm that occurred on December 14, 2006. The parameters used in our analyses are the top frequency (ftEs) and blanketing frequency (fbEs) of the Es layer as measured over São Luís, Brazil (2.33°S, 44.2°W, dip: −4.5°) by digital ionosonde. A tentative association between these parameters and X-ray data measured by sensors on board the GOES satellite was carried out. Also, we investigated the effects on the dynamics of the equatorial electrojet using magnetometer data related to the presence of these Es layers. Our analyses show that there are notable changes in the fbEs, which are characterized by the occurrence of peaks that exceed the ambient background values.
Typical sporadic E layers (Es) are very narrow (normally 2–10 km in height) layers of ionization enhancements with respect to the ambient ionization density. They are normally observed in the E -region in the height range between 90 and 130 km and are quite wide horizontally (covering tens to hundreds of kilometers). They are characterized by a large day-to-day variability and distinct features depending on the altitude and latitude where they are observed. Es layers are detected in ionograms due to reflections in the thin layer of plasma density enhancement due to long-lived metallic ions such as Fe+,Mg+,Ca+ and Na+ (Tsunoda, 2008). The layer density can be up to an order of magnitude greater than background densities, mainly because their lifetime is longer than the dominant species (O+,NO+,O2+)at the E -region heights (Kopp, 1997).
Based on HF sounders, several different types of Es have been detected and their classification is based primarily on their appearance in ionograms, taking into account the transparent, or blanketing, nature and the location. The formation mechanisms have been understood only later. The blanketing frequency Es layers (Esb) (Tsunoda, 2008) were originally classified as being of several types, and were labeled as follows: “f” (flat, a nocturnal type of Es), “l” (low, a diurnal type of Es), “c” (cusp, connected to the end of the E-layer trace), “h” (high, observed at higher altitudes), and “a” (auroral). The development of Es-a layers are strongly associated with particle precipitation at high latitudes (Sauli and Bourdillon, 2008). In a comprehensive study of middle and low latitudes, Whitehead (1989) stated that E-region ionization enhancements (Es-l, -f, -c, -h) can be formed due to vertical shear caused by opposing horizontal neutral winds. Such a shear can be driven by gravity waves (Hook, 1970; Jayachandran, 1991; Lanchester et al., 1991) or tidal motions (Chimmonas, 1971).
In the equatorial region, the most frequently-observed type of Es layer is the type “q” (equatorial). Despite the fact that this kind of signature in the ionograms does not indicate a layer in the sense of resulting in an enhancement of the electron density, and does not block signals reflected at the upper regions, it is classified as an Es layer in ionosphere research based on ionosondes (Piggot and Rawer, 1972). Indeed, the Esq layer is associated with equatorial electrojet (EEJ) current instabilities (Fejer and Kelley, 1980; Forbes, 1981). The signatures of the Esq layers in ionograms appears as a scattering of the ionosonde signal covering most of the frequency scale, and occurs during daylight time. Knecht and Mcduffie (1969) showed that the Esq can be detected over a distance of about 700 km in latitude around the magnetic equator, which agrees with the width of the equatorial geomagnetic field enhancements observed with a magnetometer. However, it must be noted that Esq layers may be observed simultaneously with other Es layers, such as the type “s” (slant), which are slants (in height) that emerge in the original layer trace (Cohen et al., 1962). A more comprehensive review on Es layers has been published by Whitehead (1970).
Studies of the presence of Es layers during periods of disturbance have being carried out by Batista and Abdu (1977) at low latitudes in the Brazilian sector. They compiled the total frequency (ftEs), and the blanketing frequency (fbEs), of Es layers during several magnetic storms during the solar cycle 20 and stated that fbEs show a well-defined enhancement of appreciable magnitude, observed 1–3 days after the Sudden Commencement (SC) of the storms. Also, Afraimovich et al. (2001) established that extreme solar flares may cause sudden changes in ionospheric ionization, which can lead to global distribution changes. They show that enhancements of X-ray and UV radiation intensity, that are observed during chromospheric flares, cause sudden increases in the process of the formation of free electrons, increasing the electron density in ionospheric layers.
In the present paper, the variation of ftEs and fbEs, obtained from a digital sounder at São Luís during the days before, during, and subsequent to, the December 2006 magnetic storm, are studied. Specifically, we have studied the abnormal density enhancement in the fbEs that are discussed in terms of the magnetic activity. Furthermore, we have also checked for a direct relationship between the occurrence of flares and E layer density enhancement, during the magnetic storm, using fbEs as a proxy of foE.
We have used data from two different types of ground-based equipment installed on, and close to, the dip equator in the Brazilian sector: a digital ionosonde and two magnetometers. Data from digital sounders at São Luís, an equatorial station, were used to determine the ionospheric frequency parameters mentioned previously. It operated every 15 minutes, sending consecutive pulses in the frequency range from 0.5 to 30 MHz, in steps of 0.5 MHz.
The Earth’s magnetic field is routinely monitored by magnetometers at the two sites, São Luís-SLZ and Vassouras-VSS (3.89°S, 38.44°W, dip: −12.5), at a rate of one measurement per second. In the present analysis, we have used a one-minute average of the H -component measurements from these two sites in order to determine the strength of the magnetic effect of the EEJ current at ground level (referred to as the EEJ ground strength, for simplification). The basic treatment of the magnetic data at each station is to eliminate outlier values from the measured components, based on a 3rd-order polynomial fitting. Thereafter, the five quietest days in a month are chosen and their local midnight values averaged. Then, the station H -component variations are normalized to the difference between the H -component values and the mean local midnight values for the five quietest days, providing the Δ H for that corresponding station. Finally, the variation of the EEJ ground strength (Δ HSLZ − Δ HVSS) is estimated by taking the difference between the ΔH values at a station at the dip equator (ΔHSLZ) and that at a station nearby the dip equator, but outside the EEJ influence (ΔHVSS). A more detailed explanation of the magnetic data treatment, and about the use of the two stations, can be found in Denardini et al. (2009).
In order to investigate the probable cause of the observed enhancements, we looked for solar flare occurrences that matched with the time of occurrence of such enhancements at the E-region heights. Therefore, we used measurements of X-ray radiation in the range 1.0–8.0 Å, made by X-ray Sensors (XRS) of the GOES 10 satellites (Geostationary Operational Environmental Satellites) located at 6.6 Earth’s Radii (Aschwanden, 1994), to identify solar flare signatures (Taylor and Watkiks, 1970).
3. Results and Discussions
The typical behavior of ftEs and fbEs is characterized by enhancements during the morning period after about 9 UT up to maximum values attained at around 15 UT. They then decrease to quiescent values after about 21 UT. The baseline of ftEs remains in the range between 4 and 6 MHz during the selected days. Top values of ftEs are about 11 MHz at 13 UT. However, on December 15 and 16, the ftEs maximum peak value reached 15 MHz (identified by red circles in the middle panel of Fig. 1). The fbEs baseline remained around 1.5 MHz in the morning period. After around 10 UT, this parameter rose to approximately 4 MHz and started decreasing to the baseline value at 21 UT. Nevertheless, it is possible to identify peaks in the fbEs that are superposed on this behavior. These significant increases of fbEs are identified by arrows with capital letters A–E. All these peaks are observed during the recovery phase of the magnetic storm that started on December 14. The first and second peaks (A and B) reached ∼5 MHz at 21 UT. At around 18 UT on December 16, the fbEs reached 7.5 MHz (C). On December 17, there occurred two peaks. The first (D) occurred during the morning (11 UT) and reached 3.5 MHz. The other peak occurred at 21 UT (E) and achieved 5 MHz. Besides the occurrence of these peaks, a different type of Es layer was detected during these observations.
The occurrence of these peaks during the recovery phase, and the simultaneous observations of different types of Es-layers, agrees with previous work by Batista and Abdu (1977). They reported a study from geomagnetic storms in 1973 and in 1975, in which fbEs reached values greater than 7 MHz 2–3 days after the storms. At that time, they had classified some sporadic layers as being type “a” and, after a careful study of winds and recombination rate, they associated the increase of fbEs to particle precipitations from the Van Allen Radiation Belt. In most of the cases in the present study, we also classified the different types of Es layer observed with the occurrence of the peak as an “a” type. However, we associated the Esa in our study with the wind shear superposed on the Gradient-Drift instability, instead of the precipitation of particles (Abdu, private communication, 2010).
In fact, Reddy and Devasia (1981) and Devasia et al. (2006) have studied the dependence of the appearance of sporadic layers at an equatorial latitude on the cycle ofsolar activity. Denardini et al. (2009) show the occurrence of an Es layer associated with CEJ for the same region as the present study. All the events that were reported during CEJ events occurred under magnetically quiet conditions. The appearance of the Esa layer in our case occurred during a magnetic storm, however. The Dst index was around −70 nT.
During the recovery phase of the geomagnetic storm, the primary zonal electric field may be reversed to westward during the day, due to the ionospheric disturbance dynamo (IDD). Richmond et al. (2003) and Huang and Chen (2008) have shown that the disturbed zonal electric field caused by the IDD may not reverse the electric field during daytime, but it can drastically reduce it instead. Also, a sudden northward turning of the B z component of the solar wind may cause overshielding in the planetary electric field and create a Prompt Penetration Electric Field (PPEF), which may reverse, or reduce, the primary zonal electric field. However, PPEFs have a duration of about 30 min to 1 h, and the SIED event we showed in the graph of the Fig. 3 lasted for almost 3 h.
Therefore, the cause of the appearance of this sporadic layer, which is related to the sudden increase in the electron density in the E region, is still an open question, although we tend to attribute it to a wind shear mechanism superposed on the Gradient-Drift instability. Moreover, due to the similar characteristics between our example and that reported by Reddy and Devasia (1981) and Devasia et al. (2006), we believe that the appearance of the sporadic layer results from the drastic reduction of the EEJ electric field, which we suppose is associated with neutral winds added to the IDD condition. Hence, a downward polarization electric field caused by the wind shear should be responsible for inverting the normal upward electric field in a constraint height range, contributing to the observed drastic reduction of the EEJ ground strength. Also, plasma irregularities in the upper portion of the sporadic layer may be detected (Crochet et al., 1979; Denardini et al., 2009).
In the present analysis, we also consider the influence on the X-ray radiation due to solar burst events. The bottom panel of Fig. 1 shows that the variation of the X-ray solar radiation intensity follows a similar behavior to the solar-cycle activity, which results in periodic disturbances in the Earth’s atmosphere and they are divided in the classes. Indeed, we have checked several cases of X-ray solar bursts associated with strong magnetic storms during solar cycle 23 (not shown here), including the present case. Most of the X-ray peaks (≈65%) caused an absorption of radio waves with wavelengths compatible with those for E -region soundings. This radio wave absorption is attributed to Sudden Ionospheric Disturbances (SID), which occurred at the D-region height (Sahai et al., 2007), thereby not allowing us to observe the upper regions. Therefore, despite the fact that characteristics of high-intensity flares (X class) can be noted on the X-ray measurements at the GOES-10 satellite, we were not able to prove that these intensifications may affect the parameters of the Es layer directly.
We have sought to identify the effect of solar activity on the density enhancement of Es-layers observed at São Luís-MA, Brazil, during disturbed periods. In general, the ftEs is characterized by a typical diurnal variation, when it does not exceed 13 MHz. However, we have shown that the ftEs reached peak values of 15 MHz on December 15 and 16. We also showed fbEs peaks superposed on the normal daily variation of this parameter. This also occurred during the recovery phase of the magnetic storm analyzed. The peak explored in the present work was associated with the presence of a sporadic layer type “a”, the formation of which we attributed to wind shear superposed on the Gradient-Drift instability. The event, which occurred on December 15, when fbEs rose to 5 MHz, was carefully studied taking into account EEJ dynamics. On this day, the Esa layer was detected at about 115 km, between 1410 and 1540 UT, and blocked all the reflections of the ionosonde signal at the upper portions of the ionospheric profile. During this time, a CEJ is clearly identified from magnetometer measurements. Therefore, we attributed this ionization increase to conditions generated by the CEJ. In the present analysis, we also consider the influence on X-ray radiation due to solar burst events. Our previous analyses revealed that X-ray solar flux intensifications caused Sudden Ionospheric Disturbances (SID) that absorbed the radio wave at the Dregion heights, not allowing us to observe the E-region. Therefore, we were not able to prove that these intensifications may affect the parameters in the Es layer directly.
L. C. A. Resende thanks Capes/MEC for her PhD fellowship and C. M. Denardini thanks CNPq/MCT (Grants 305923/2008-0 and 470553/2009-0). The authors thank NOAA for making available the GOES satellite data.
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