ANGEOAnnales GeophysicaeANGEOAnn. Geophys.1432-0576Copernicus PublicationsGöttingen, Germany10.5194/angeo-35-345-2017Ionospheric response to magnetar flare: signature of SGR J1550–5418 on
coherent ionospheric Doppler radarMahrousAymanamahrous@cern.chSpace Weather Monitoring Center, Physics Dept., Faculty of Science, Helwan
University, Ain Helwan, Cairo 11795, EgyptAyman Mahrous (amahrous@cern.ch)7March201735334535127July20168February201713February2017This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://angeo.copernicus.org/articles/35/345/2017/angeo-35-345-2017.htmlThe full text article is available as a PDF file from https://angeo.copernicus.org/articles/35/345/2017/angeo-35-345-2017.pdf
This paper presents observational evidence of frequent ionospheric
perturbations caused by the magnetar flare of the source SGR J1550–5418,
which took place on 22 January 2009. These ionospheric perturbations are
observed in the relative change of the total electron content (ΔTEC/Δt) measurements from the coherent ionospheric Doppler radar
(CIDR). The CIDR system makes high-precision measurements of the total
electron content (TEC) change along ray-paths from ground receivers to low
Earth-orbiting (LEO) beacon spacecraft. These measurements can be integrated
along the orbital track of the beacon satellite to construct the relative
spatial, not temporal, TEC profiles that are useful for determining the
large-scale plasma distribution. The observed spatial TEC changes reveal many
interesting features of the magnetar signatures in the ionosphere. The onset
phase of the magnetar flare was during the CIDR's nighttime satellite
passage. The nighttime small-scale perturbations detected by CIDR, with
ΔTEC/Δt≥ 0.05 TECU s-1, over the eastern
Mediterranean on 22 January 2009 were synchronized with the onset phase of
the magnetar flare and consistent with the emission of hundreds of bursts
detected from the source. The maximum daytime large-scale perturbation
measured by CIDR over northern Africa and the eastern Mediterranean was detected
after ∼ 6 h from the main phase of the magnetar flare, with ΔTEC/Δt≤ 0.10 TECU s-1. These ionospheric
perturbations resembled an unusual poleward traveling ionospheric
disturbance (TID) caused by the extraterrestrial source. The TID's
estimated virtual velocity is 385.8 m s-1, with ΔTEC/Δt≤ 0.10 TECU s-1.
Ionosphere (ionospheric
disturbances) – radio science (radar astronomy) – solar physicsastrophysicsand astronomy (X rays and gamma rays)Introduction
Magnetars are a subclass of isolated neutron stars possessing the most extreme magnetic
fields in the universe with B>1014–1015 G (Thompson and Duncan, 1995, 1996). The magnetar model
(Thompson et al., 2002) identified the power source of these objects to be
the decay of their strong magnetic fields. Originally classified as two
distinct types of objects, anomalous X-ray pulsars (AXPs) and soft gamma
repeaters (SGRs) are currently believed to be of the magnetar class (for
reviews, see Woods and Thompson, 2006; Mereghetti, 2008). In addition to being
bright X-ray sources, SGRs and AXPs emit intense bursts in hard X-rays and soft
gamma rays at a highly unpredictable frequency with peak luminosities ranging
from 1038 to > 1047 erg s-1, which is considered as the
observational signature of magnetars (Duncan and Thompson, 1992; Kouveliotou
et al., 1998). They are known to exhibit emissions as sporadic bursts, which
are classified into three kinds according to their luminosities and
durations: giant flares, intermediate flares, and short bursts.
The X-ray source 1E 1547.0–5408, also known as SGR 1550–5418, was
discovered in 1980 with the Einstein satellite as a point source (Lamb and
Markert, 1981). The discovery of radio pulsations with a period of 2.1 s and
a period derivative of 2.3×10-11 s s-1 confirmed its AXP classification (Camilo et al., 2007). It was
identified only recently as a magnetar by Gelfand and Gaensler (2007) based
on its X-ray spectrum and infrared flux. The dipole surface magnetic field
strength and characteristic age are estimated to be about 3.2×1014 G and 0.69 kyr, respectively (Dib et al., 2012). These features
make this object relatively young and classify it as one of the fastest known
rotating magnetars.
In early October 2008, both the Swift Burst Alert Telescope (BAT) and the
Fermi Gamma Ray Burst Monitor (GBM) were triggered by numerous bursts from
the source SGR 1550–5418 (Israel et al., 2010; von Kienlin et al., 2012).
The source entered a second, even more active phase on 22 January 2009,
during which a large number of bursts were observed by several satellites, as
detected by Swift (Gronwall et al., 2009), Fermi GBM (Connaughton and Briggs,
2009; von Kienlin and Connaughton, 2009), Konus-Wind (Golenetskii et al.,
2009), and RHESSI (Bellm et al., 2009). BAT was triggered on 22 January 2009
at 01:32:41 UT and the first X-Ray telescope (XRT) observation on the Swift
satellite began about 50 min later. The peak of the persistent emission
occurred ∼ 6 h after BAT was first triggered. This was accompanied by a
hardening of the spectrum in the 1 × 10 keV band (see a summary of
the observations in Table 1 in Scholz and Kaspi, 2011). The wide-band all-sky
monitor (WAM) onboard the Suzaku satellite detected at least 254 bursts in
the 0.16–6.2 MeV band over the period of 22 January 2009 00:57–17:02 UT
from the direction of the source. One of these bursts, which occurred at
06:45:13 UT, produced the brightest fluence in the 0.5–6.2 MeV range, with
an averaged 0.16–6.2 MeV flux and extrapolated 25 keV–2 MeV fluence of
about 1×10-5 and about 3×10-4 erg cm-2,
respectively (Terada et al., 2009). Mereghetti et al. (2009) reported on the
observations obtained by the INTEGRAL satellite on 22 January 2009 with the
emission of hundreds of bursts in a time span of a few hours, starting at
02:46 UT. The peak of the bursting rate occurred around 06:48 UT, when more
than 50 bursts were recorded in 10 min. The total fluence measured from the
125 bursts emitted from 04:30 to 07:00 UT was 5.2×104 erg cm2 (25 keV–2 MeV). This activity was observed by
several high-energy missions, creating a good opportunity for investigating
the broadband spectra of magnetar short bursts and intermediate flares in
detail. Broadband spectral properties have been reported by several authors
(e.g., van der Horst et al., 2012; Lin et al., 2012; Younes et al., 2014).
Earth's ionosphere can be thought of as a gigantic detector that responds to
the ionizing radiation emitted through high-energy astrophysical phenomena
without interruption such as Earth occultation (Mondal et al., 2012). Soft
X-ray emissions from solar flares are the more common sources of ionospheric
disturbances, which can be monitored using the very low frequency (VLF) technique (Bracewell and
Straker, 1949; Thomson et al., 2005; Pacini and Raulin, 2006; Raulin et al.,
2006, 2010). In addition to these solar–terrestrial events, the lower ionosphere is
also affected by high-energy photons (X-rays and gamma rays) from
extraterrestrial sources like gamma ray burst (GRB) and SGR (Inan et al., 1999, 2007; Tanaka
et al., 2010). Cosmic gamma rays play an important role in ionizing the
neutral atmosphere through electromagnetic cascading (Mahrous and Inoue,
2002). The ionospheric disturbance caused by a cosmic gamma ray burst was
first reported by Fishman and Inan (1988). It suggested that gamma rays
deposit their energies in the lower ionosphere, abnormally ionize the neutral
atmosphere there, and modify the electron density height profile. So far the
detection of ionization excesses by using VLF observations has only been reported
for four extraterrestrial events (for summary see Raulin et al., 2014).
Map shows the solar nighttime and daytime conditions (dark blue and
light blue areas, respectively) on 22 January 2009 at 00:52 UT, the
projection on the surface of the Earth of the Sun's position (yellow circle),
moon's position (white circle), and the sub-flare point (red star). The black
rectangle defines the field of view of CIDR, as indicated by the satellite
tracks in Fig. 2.
Many authors observed and reported the first observation of the source SGR
J1550–5418 through VLF perturbation on 22 January 2009. The South
America VLF Network (SAVNET; Raulin et al., 2009) clearly showed sudden
amplitude and phase changes at the corresponding times of eight of these X-ray
bursts (Tanaka et al., 2010). Mondal et al. (2012) found convincing evidence
that the lower ionospheric height went down significantly by about 15 km
during that event. They also computed the evolution of the electron number
density of the ionosphere due to that event and found that the ionosphere was
becoming increasingly charged due to repeated bombardment of the high-energy
radiations. Raulin et al. (2014) detected an ionospheric disturbance during
the event, which was revealed by the simultaneous phase and amplitude records
from two VLF propagation paths between the transmitter NPM (Hawaii) and the
receivers ROI (Brazil) and EACF (Antarctic Peninsula). Although the
previously mentioned authors have specifically studied the temporal sudden
ionospheric disturbance (SID) due to the magnetar flare, there was no
information about the spatial ionospheric perturbations, sometimes called the
traveling ionospheric disturbance (TID), caused by the event.
The TIDs are understood as plasma density fluctuations that propagate
through the ionosphere at an open range of velocities and frequencies. The
trends of such fluctuations can be seen in most of the ionosphere measurement
techniques (Hernández-Pajares, et al., 2006). However, the exact generation
mechanisms of TIDs, such as Joule heating and Lorentz force (e.g., Oyama and
Watkins, 2012) are only poorly understood because of several interaction
mechanisms with electric and magnetic fields as well as thermospheric winds.
In this paper, we introduced the spatial, not temporal, ionospheric
disturbance during the gigantic extraterrestrial event from the source SGR
J1550–5418 that took place on 22 January 2009. The unusual TID parameters
due to that magnetar flare, such as direction and speed are estimated. The
paper is divided into four sections. We describe the instrumentation in Sect. 2,
the discussion in Sect. 3, and finally the conclusions in Sect. 4.
Instrumentation
The Ionospheric Tomography Network of Egypt (ITNE) is a chain of passive
UHF–VHF (ultra-high-frequency) receivers, known as coherent ionospheric Doppler radars (CIDRs). The
first ITNE CIDR was installed in May 2008 at Helwan (geographic latitude
29.9∘, longitude 31.3∘) (Mahrous et al., 2010). CIDR
receivers make high-precision (on the order of 104 TECU s-1)
(1 TEC unit (TECU) = 1016 electrons m-2) measurements (where
TEC is total electron content) between rays underneath low Earth-orbiting (LEO) spacecraft. Each
receiver measures the Doppler shift in 150 and 400 MHz signals at a 1 s
data rate. A linear combination of these Doppler shifts reveals relative change of the total electron content (ΔTEC/Δt). These measurements can be integrated along the orbital track
of the beacon satellite, flying between 700 and 1100 km, to construct the
relative TEC profile at a 1 s data rate (relative to an unknown integration
constant). The derived TEC values are useful for determining the large-scale
plasma distribution, but the ΔTEC/Δt data are more sensitive
to small-scale phenomena and wave activity. The CIDR system has many
advantages over the Global Navigation Satellite System (GNSS). Because CIDR's
LEO spacecraft fly at lower orbits, with an altitude range of 521 to 1158 km,
the measured TEC is purely ionospheric, as opposed to GNSS TEC measurements,
which have a minor plasmaspheric component. In addition, LEO satellites cross
the receiver's field of view in less than 15 min; therefore, the structures
with ΔTEC/Δt are usually treated as spatial not temporal
variations. CIDRs and their measurements are discussed in greater detail by
Garner et al. (2008, 2009).
Discussion
This study examines five satellite passes observed by CIDR during the SGR
J1550–5418 burst on 22 January 2009. We add one additional pass to show the
recovery phase of the magnetar flare on 23 January 2009. The space weather
data show that 22 January 2009 was a geomagnetically quiet day. The total Kp
index was 2.0 (http://wdc.kugi.kyoto-u.ac.jp/kp/index.html), and the
F10.7 cm solar radio flux was 70.1 × 1022 W m-2 Hz
(http://eng.sepc.ac.cn/F107Index.php). Figure 1 shows the solar
terminator during the SGR burst on 22 January 2009 at 00:52 UT. The dark and
light blue areas indicate the regions under night time
conditions and under solar illumination, respectively. The white and yellow circles
represent the projections of the moon and the Sun, respectively, on the Earth's
surface. The point on the Earth directly beneath the flare (sub-flare point)
was located at 54.3∘ S, 14.0∘ E (Tanaka et al., 2010), and
its position is defined by a red star. The black rectangle shows the field of
view of CIDR, corresponding to the satellite tracks in Fig. 2. The onset phase
of the magnetar flare was during the CIDR's nighttime satellite passage.
Figure 2 shows a set of maps of the CIDR's passing satellites over northern
Africa and the eastern Mediterranean region. The satellite tracks are shown
in solid lines and the ionospheric pierce point (i.e., the intersection of
the receiver satellite line of sight with the ionosphere, assumed as a thin
layer at a fixed height) is shown in thick lines on the map, respectively. The CIDR's
passing satellites began at 01:21 UT on 22 January 2009 and ended on
23 January 2009 at 04:10 UT. Half of the satellite passes were during the
local nighttime (indicated by black filled circles in the bottom right
corner of each map), while the rest were during the daytime (open white
circles). The F region pierce point is set to 350 km and the obliquity
factor is only used for azimuth angles within 30∘ of an overhead
pass. The geomagnetic equator is offset 7–8∘ north of the geographic
equator in the Egyptian sector.
Figure 3 shows the vertical total electron content (VTEC) along the satellite
tracks shown in Fig. 2 as a function of the geographic latitude. The VTEC
latitudinal distribution at the 350 km intercept resembles a logarithmic
profile. The complete profile of the equatorial anomaly is shown in panels a,
c, and d. The gradual reduction of VTEC from 22 to 30∘ geographic
latitude (from 15 to 23∘ magnetic latitude) suggests that radio rays
are passing through the equatorward side of the northern equatorial fountain
peak (see Mahrous et al., 2010, for details). The total VTEC gradient was
slightly larger than 5×10-2 TECU per degree, as
measured by the OSCAR 25 satellite overpass. A larger TEC gradient is
observed in those local sunrise passes (panels c, d, and e) than was seen
during the previous nighttime passes (panels a, b, and f). This enhancement is
over 3 times larger than the enhancement observed during the morning pass.
Set of maps show the satellite track (thin line) and the F region
(thick line) pierce points over northern Africa and the eastern Mediterranean.
The local night and day times during the satellite passes are indicated by
black filled and white open circles at the bottom right corner of each map.
The CIDR passes (alphabetically sequenced from a to f) were
detected on 22 January 2009 at 01:21, 02:56, 12:48, 14:36, 15:44 UT and on
23 January 2009 at 04:10 UT, respectively.
VTEC versus geographic latitude of the F region intercept. The
passage time corresponds to the same panels (alphabetically sequenced)
in Fig. 2.
ΔTEC/Δt versus geographic latitude of the F region
intercept. The passage time corresponds to the same panels
(alphabetically sequenced) in Fig. 2. The vertical red dashed lines show the
disturbance upper limits during the main phase of the magnetar's flare.
Figure 4 shows the relative change in the total electron content ΔTEC/Δt along the corresponding satellite tracks shown in Fig. 2 as a
function of the geographic latitude. The VTEC latitudinal distribution at the
350 km intercept resembles a logarithmic profile. The first CIDR passage
(Fig. 4a) showed that the ionosphere was remarkably smooth on 22 January 2009
at 01:21 UT. Both BAT and INTEGRAL were triggered with the emission of
hundreds of bursts in the band (25 keV–2 MeV) from the source SGR
1550–5418 at 01:32:41 and 02:46 UT, respectively (see Scholz and Kaspi,
2011; Mereghetti et al., 2009). Nearly 10 s after INTEGRAL
was triggered, the CIDR started to detect small-scale ionospheric
disturbances over the eastern Mediterranean between 30.5 and 41∘
geographic latitude at 02:56 UT (Fig. 4b), with ΔTEC/Δt≥ 0.05 TECU s-1. The peaks of the bursting rate detected by BAT,
Suzaku, and INTEGRAL occurred around 07:32:41, 06:45:13, and 06:48 UT,
respectively. These daytime emissions affected the lower ionosphere with
hundreds of bursts in the band 1 kev–6.2 MeV (Tanaka et al., 2010; Mondal
et al., 2012; Raulin et al., 2014). A high level of ionospheric perturbations
were detected by CIDR over northern Africa and the eastern Mediterranean
∼ 6 h later between 23 and 28∘, with ΔTEC/Δt≤ 0.10 TECU s-1 at 12:48 UT (see Fig. 4c).
The visual examination of this passage found a structure in the ΔTEC/Δt profile similar to the wave-like structure that occurred between 29 and
42∘. It is worthwhile to mention that this is a signature of gravity
waves created over the Anatolian plateau. This scenario is suggested by
Garner et al. (2011), who verified that these kinds of ionospheric
perturbations are associated with the orographic lift of the atmosphere over
the Anatolian region, possibly through the propagation of the upward gravity
waves. The amplitude of these waves is ≥ 0.01 TECU s-1,
indicating that they would not generally be visible during more active
ionospheric conditions.
The ionospheric perturbation continued, and even expanded, to much higher
latitudes up to 35∘ (see Fig. 4d). These structures resembled the
poleward TIDs observed between 23 and 36∘ at 14:36 UT. The upper
latitude limit of the ionospheric disturbance detected during the main phase
of the magnetar flare is indicated by the red dashed vertical lines in
Fig. 4c and d. The comparison between the two upper limits defines the growth
of the disturbed area, which expanded from 28 to 35∘ within 1.8 h.
The TID's estimated virtual velocity is 385.8 m s-1, with ΔTEC/Δt≤ 0.10 TECU s-1. The descending phase of the
ionospheric perturbation observed between 33 and 37.5∘ continued
until 15:44 UT (Fig. 4e), then totally recovered on 23 January 2009 at
04:10 UT (Fig. 4f).
Conclusions
This paper presents convincing evidence that the source SGR J1550–5418
repeatedly caused significant ionospheric disturbances during the burst onset
phase of the magnetar flare. The space weather data show that 22 January 2009
was a geomagnetically quiet day, with no effects of solar flares and
geomagnetic storms on the ionosphere. The repeated bombardment of the high-energy radiations by that source was enough to enhance the electron number
density of the ionosphere as computed by Mondal et al. (2012). The spatially
relative TEC changes observed by the CIDR system reveal many interesting
features of the magnetar's signature in the ionosphere. The northern
equatorial fountain peak and gravity waves created over the Anatolian plateau
are two ordinary phenomena resolved from VTEC latitudinal distribution. The
onset phase of the magnetar flare was during the CIDR's nighttime satellite
passage.
The nighttime small-scale perturbations detected by CIDR, with ΔTEC/Δt≥ 0.05 TECU s-1, over the eastern Mediterranean
on 22 January 2009 were synchronized with the onset phase of the magnetar
flare and were consistent with the emission of hundreds of bursts from the
source,
as detected by BAT and INTEGRAL. It is well known that only the stronger
bursts can be detected during daytime ionospheric conditions. In the present
case, the emitted bursts would have produced an ionospheric disturbance large
enough to be detected by CIDR during the daytime. The maximum large-scale
perturbation was detected by CIDR, with ΔTEC/Δt≤ 0.10 TECU s-1, over northern Africa and the eastern Mediterranean
during the daytime ∼ 6 h after the main phase of the magnetar flare.
These ionospheric perturbations resembled an unusual poleward TID caused by
an extraterrestrial source. The TID's estimated virtual velocity was
385.8 m s-1, with ΔTEC/Δt≤ 0.10 TECU s-1. The direction of the TID towards the North Pole
suggests the opposite location of the ionizing source, which is in harmony
with the sub-flare point (see Fig. 1). The proposed creation mechanism is
similar to joule or particle heating, except that the ionizing radiation is
X-ray emitted by the source SGR 1550–5418.
In general, CIDR observations appear as an interesting diagnostic tool of
high-energy astrophysical bursts emitted by extraterrestrial sources. For
magnetars, their ionospheric response complements their detection in space,
in particular when space observations are not available due to detector
saturation or Earth occultation.
Short-time data are available. Kindly contact the author.
The author declares that he has no conflict of interest.
Acknowledgements
The ITNE CIDRs were constructed under the Defense University Research
Instrumentation Program grant FA9550-04-1. The deployment of ITNE was
supported by a US–Egypt Joint Science and Technology Fund and the National
Oceanic and Atmospheric Administration contract RA133D-07-SE-3966. The author
also wishes to express his thanks to the anonymous reviewers and Mark Moldwin
for their useful comments and review of the paper. The topical editor, E. Yizengaw, thanks the three anonymous
referees for help in evaluating this paper.
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