Midlatitude ionospheric F2-layer response to eruptive solar events-caused geomagnetic disturbances over Hungary during the maximum of the solar cycle 24: a case study K. A. Berényi *,1,2, V. Barta 2, Á. Kis 2 1 Eötvös Loránd University, Budapest, Hungary 2 Research Centre for Astronomy and Earth Sciences, GGI, Hungarian Academy of Sciences, Sopron, Hungary Published in Advances in Space Research Abstract In our study we analyze and compare the response and behavior of the ionospheric F2 and of the sporadic E-layer during three strong (i.e., Dst <-100nT) individual geomagnetic storms from years 2012, 2013 and 2015, winter time period. The data was provided by the state-of the art digital ionosonde of the Széchenyi István Geophysical Observatory located at midlatitude, Nagycenk, Hungary (IAGA code: NCK, geomagnetic lat.: 46,17° geomagnetic long.: 98,85°). The local time of the sudden commencement (SC) was used to characterize the type of the ionospheric storm (after Mendillo and Narvaez, 2010). This way two regular positive phase (RPP) ionospheric storms and one no-positive phase (NPP) storm have been analyzed. In all three cases a significant increase in electron density of the foF2 layer can be observed at dawn/early morning (around 6:00 UT, 07:00 LT). Also we can observe the fade-out of the ionospheric layers at night during the geomagnetically disturbed time periods. Our results suggest that the fade-out effect is not connected to the occurrence of the sporadic E-layers. Keywords: ionospheric storm, geomagnetic disturbance, space weather, midlatitude F2-layer, sporadic E-layer *Corresponding author. Tel.: +3670 3198192. E-mail adresses: [email protected] (K. Berényi), [email protected] (V. Barta) [email protected] (Á. Kis). 1 1. Introduction Perturbations and changes in the ionosphere are caused by multiple effects. The most significant and well documented effects are those that occur during periods of global geomagnetic storms (Mendillo and Narvaez, 2010). Geomagnetic storms are time periods that are characterized by significantly enhanced geomagnetic activity that are driven by disturbances in the solar wind. Previous studies found two kinds of events which can induce geomagnetic storms (e.g. (Denton et al., 2009; Gonzalez et al., 1994; Tsurutani et al., 2006): 1. Coronal Mass Ejections (CME) and 2. Corotating Interaction Region (CIR)/ High-Speed Solar wind streams (HSS). During the maximum of a solar cycle the CME-related geomagnetic storms are lot more common. The most intense geomagnetic storm of the solar cycle 24 maximum (examined in this study) is also connected to CME. Several definition exist to determine a geomagnetic storm and its magnitude. Previous studies have typically identified and characterized the storm events considering the magnitude of geomagnetic indices such as Kp or Dst (Kane and Makarevich, 2010). Gonzalez et al. (1994) defined a geomagnetic storm as a period in which the Dst-index exceeds some key thresholds, as follows: events with a Dstmin between −30 nT and −50 nT are weak storms, between −50 nT to −100 nT are moderate storms, and events with a Dstmin less than −100 nT can be considered as intense storms. For storms with Dstmin < -300 nT the term super-storm is used (Burešová and Laštovička, 2007). There is another subdivision of intense storms: the strong (−200 ≤ 퐷푠푡푚푖푛 ≤ −100 푛푇) , very strong (−350 ≤ 퐷푠푡푚푖푛 ≤ −200 푛푇) , and great (퐷푠푡푚푖푛 ≤ −350 푛푇) storms (Danilov, 2013). A geomagnetic storm generate a so-called ionospheric storm in the ionosphere, which has a similar evolution and phases as a geomagnetic storm, but with a faster course. The scientific community agreed, that only the large geomagnetic storms induce global ionospheric storms (Buresova et al., 2014). The general course of the midlatitude ionospheric F2-layer response to geomagnetic storms has been described by Rishbeth and Field (1997) and summarized by Prolss (2004) in the recent years. The initial phase starts with enhanced peak electron density, which last for few hours after the SSC of the geomagnetic storm. During the main phase, which lasts for a day or more, the electron density (Ne) is decreased compared to quiet day values (negative storm phase), but sometimes it is enhanced (positive storm phase). Lastly the recovery phase of the storm can last from hours to days. The midlatitude ionosphere provides a good location to study different, comparable effects caused by multiple mechanisms. The impacts can origin from lower and higher latitudes too. Thus although the investigation of the ionosphere at midlaltitude is fraught with difficulties, it can increase our general understanding of the background physical processes and how they act paralelly. There are several processes that have to be taken into consideration during the examination of the midlatitude ionosphere, as follows: photo-production, chemical loss, and transport by thermal expansion, neutral winds, waves, tides and electric fields of internal and external origin (Mendillo and Narvaez, 2009). All of these come up during disturbances. Further influential factors, that need also to be taken into account during ionospheric studies (Immel and Mannucci, 2013; Mendillo and Narvaez, 2010, 2009): local time of SSC, the magnitude of the geomagnetic storm (based on Kp- and Dst-index values), the type of the geomagnetic storm 2 (CME-related or CIR/HSS-related), the phase of the solar cycle, the geomagnetic latitude and longitude of the station and the season of the year (i.e., winter or summer). The “smoothed” picture of the F-region changes is widely accepted since the end of the 1990s (see reviews of Prölls, 1995; Buonsanto, 1999; Mendillo, 2006), but the global distribution of ionospheric storm effects is rather complicated and differs considerably from storm to storm (Danilov, 2013). So according to the observations it can be concluded, that both a significant electron density decrease and an anomalous density increase can occur in the F2-layer over a course of a magnetic storm (Buresova et al., 2014). Also it has been observed, that there are cases when significant magnetic disturbances did not followed by ionospheric disturbance, or vice versa (Buresova et al., 2007; Field et al., 1998; Li et al., 2012). The analysis of the sporadic E-layer (Es-layer) can be informative during intense storm event. The blanketing behavior of this layer can be in some cases responsible for the fade out of upper ionospheric layers like F1- and F2-layer. The formation of this layer is connected to the penetrated meteoric metallic and molecular ions into the E-layer heights (90-120 km), which generate a layer thanks to the wind shear of neutral waves (Pietrella and Bianchi, 2009; Whitehead, 1989). The Es-layer is formed by “patches” in the E-layer with larger electron density. The lifetime of these Es-layers are found to be between 2-10 h (see Harwood, 1961; Tanaka, 1979; Houminer et al., 1996). The solar and geomagnetic influence on the midlatitude Es-layer parameters has been studied in the last decades (Baggaley, 1985, 1984; Maksyutin and Sherstyukov, 2005; Pietrella and Bianchi, 2009; Whitehead, 1989, 1970): it was shown, that positive, negative and also no correlation of Es-layer parameters can be expected in the function of geomagnetic activity. The correlation may vary with intensity of Es-layer, season and time of the day (Maksyutin and Sherstyukov, 2005). Further investigations are required to understand the behavior of this layer. In this preliminary study we focused on the determination of the occurring ionospheric storm patterns in the F2-layer at a Hungarian ionosonde station during the most intense geomagnetic storm event of the solar cycle 24 maximum. It is important to mention that this current solar cycle is weaker than the previous ones therefore we do not have so much strong CME impacts which can generate intense geomagnetic storms. Previous studies mostly used superposed epoch analysis (SEA) or median values to examine the ionospheric variation and the magnitude of the changes caused by various effects (Kane and Makarevich, 2010; Mendillo and Narvaez, 2010). The main goal of our investigation is to compare/supplement the previous results (of e.g. Mendillo and Narvaez, 2009, 2010) that were based on statistical studies. In our study we analyze individual events in order to observe the specific patterns because the averaging tends to eliminate the individual signature of the event. Besides the verification of the previous studies and the accepted general features occurring during intense ionospheric storm events are important and necessary when we would like to develop the accuracy of the space weather prediction models. The unique/specific features that appear during an individual storm can be important and can give us new information about the processes. 2. Analysis methods The data provided by the state-of the art ionosonde located at the Széchenyi István Geophysical Observatory in North-West Hungary, in Nagycenk (IAGA code: NCK) was used for the analysis. Its McIllwain number is L =1.9 therefore this station is situated at optimal 3 (magnetic) latitude to observe the ionospheric processes in a midlatitude region. The other important information about the station: geomagnetic latitude: 46,17°, geomagnetic longitude: 98,85°, inclination (dip angle): 66,83°. The ionosonde at the NCK observatory is a VISRC-2 type ionosonde (for a detailed description of the ionosonde see the article by Sátori et al., 2013). The relevant ionospheric parameters were determined after a careful manual evaluation of the ionograms. During the investigation we focused on the foF2 parameter because this parameter indicates the maximum plasma (electron) density in the F2-layer. We also examined the foEs parameter (maximum electron density of sporadic E-layer) and h’F2 parameter (virtual height of the F2-layer) in order to study the ionospheric processes during geomagnetic storms. The studied time interval covers three years of maximum phase of the solar cycle 24, i.e., from 2012 to 2015, in winter time period.