ANGEOAnnales GeophysicaeANGEOAnn. Geophys.1432-0576Copernicus GmbHGöttingen, Germany10.5194/angeo-33-525-2015Direct observations of blob deformation during a substormIshidaT.ishida.tetsuro@nipr.ac.jpOgawaY.KadokuraA.HosokawaK.OtsukaY.https://orcid.org/0000-0002-3098-3859National Institute of Polar Research, Tokyo 190-8518, JapanDepartment of Polar Science, SOKENDAI (The Graduate University for
Advanced Studies), Tokyo 190-8518, JapanDepartment of Communication Engineering and Informatics, University of
Electro-Communications,Tokyo 182-8585, JapanSolar-Terrestrial Environment Laboratory, Nagoya University, Nagoya
464-8601, JapanT. Ishida (ishida.tetsuro@nipr.ac.jp)6May201533552553014October201425February201517April2015This 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/33/525/2015/angeo-33-525-2015.htmlThe full text article is available as a PDF file from https://angeo.copernicus.org/articles/33/525/2015/angeo-33-525-2015.pdf
Ionospheric blobs are localized plasma density enhancements, which are
mainly produced by the transportation process of plasma. To understand the
deformation process of a blob, observations of plasma parameters with good
spatial–temporal resolution are desirable. Thus, we conducted the European
Incoherent Scatter radar observations with high-speed meridional scans
(60–80 s) during October and December 2013, and observed the temporal
evolution of a blob during a substorm on 4 December 2013. This paper is the
first report of direct observations of blob deformation during a substorm.
The blob deformation arose from an enhanced plasma flow shear during the
substorm expansion phase, and then the blob split into two smaller-scale
blobs, whose scale sizes were more than ∼ 100 km in latitude.
Our analysis indicates that the Kelvin–Helmholtz instability and
dissociative recombination could have deformed the blob structure.
Localized plasma density enhancements are often produced in the
high-latitude ionosphere by the transportation process of plasma or particle
precipitations. Among such plasma density enhancements, structures enhanced
by a factor of 2–10 above the background density, and with horizontal
dimensions ranging from ∼ 100 to 1000 km, are generally called
patches in the polar cap, or blobs outside of the polar cap (e.g., Tsunoda,
1988; Crowley et al., 2000). The blobs are generally categorized into the
following three types: (1) boundary blobs, (2) subauroral blobs, and (3)
auroral blobs. Boundary blobs are regarded as a proxy of the equatorward
auroral boundary. Subauroral blobs resemble boundary blobs but are found in
the trough region. In contrast, auroral blobs are observed in the auroral
oval, and they appear to be localized in longitude when compared with
boundary and subauroral blobs. Blobs/patches are important research subjects
because they turn into ionospheric irregularities through some sort of
deformation processes, and these irregularities affect radio wave
propagation and scintillation of Global Navigation Satellite System (GNSS)
signals (e.g., Moen et al., 2013; Jin et al., 2014). Blobs/patches are known
to be created by temporal/spatial changes in the convection pattern (e.g.,
Livingstone et al., 1982; Crowley et al., 2000; Hosokawa et al., 2010a).
Besides, high-speed plasma flow can also deform them through the enhanced
dissociative recombination process (Valladares et al., 1994). Thus, it is
clear that background plasma convection plays an important role in blob
deformation. However, detailed blob behavior (deformation/splitting) under
highly variable plasma convection during substorms is still unclear because
of the lack of adequate observations. Therefore, new techniques that can
follow variations of plasma parameters (e.g., density, velocity, and
temperature) in detail are highly desirable.
In past studies, large-scale ionospheric density structures including blobs
have been investigated using ground-based global positioning system (GPS)
receivers to obtain TEC (total electron content) maps (e.g., Coster et al.,
2003; Foster et al., 2005; Hosokawa et al., 2010b; Zhang et al., 2013). The
TEC maps can provide visualizations of the temporal evolution of the
large-scale ionospheric density structure, but they cannot resolve the
small-scale structure because of their limited spatial–temporal resolution
(grid resolution of 1∘ for the latitude and 2∘ for the
longitude, temporal resolution of 5 min). Furthermore, to study the blob
deformation process in detail, we need to know not only the plasma structure
but also the motion of the plasma convection with sufficient temporal
resolution. Therefore, we conducted the European Incoherent Scatter (EISCAT)
radar observations with high-speed meridional scans (grid resolution of
∼ 0.1–0.5∘ for the latitude, temporal
resolution of 60–80 s) during October and December 2013. In this campaign,
we encountered a convected blob inside the high-latitude trough in the
pre-midnight to dusk region. The blob was deformed during a substorm. In
this study, we examine the temporal evolution of the blob structure using
the EISCAT data and other complementary data such as TEC maps and
ground-based magnetometer data.
The TEC map after onset of the substorm. The top left panel is an
overview of the polar ionosphere around onset time, and the top right panel
enlarges the black box shown in the top left panel; these data represent the
TEC variation around the EISCAT FOV. The color scale runs from high-electron
density (red) to low-electron density (blue). The dashed black line in the
top left panel represents the solar terminator, where SZA = 90∘. The black rectangle in the top right panel indicates the EISCAT FOV, and
the dashed curving line indicates the boundary of the high-latitude trough.
(a–f) indicate a time sequence of the horizontal shape of the density structures
around the EISCAT FOV. The median filter was applied to each TEC map.
Observations
The high-speed meridional scans of the EISCAT UHF radar take 60–80 s to
scan elevation angles from 25 to 89∘. The EISCAT UHF
radar is located in Tromsø, northern Norway (69∘35′ N,
19∘14′ E; invariant latitude: 66∘12′ N), and this
observatory collects information on the following ionospheric plasma
parameters: electron density (Ne), ion temperature
(Ti), electron temperature (Te), and
line-of-sight ion velocity (Vi(LOS)). The observations were
conducted as part of the peer-reviewed program (PP) and Japanese Special
Program (SP) during October and December 2013, and we obtained a total of
nine events that each lasted for a period of 4 h (14:00–18:00 UT,
MLT≃UT+2.2h). To date, the EISCAT observations of the
blob have been conducted mainly by Common Program 3 (CP-3) scans, which take
∼ 30 min for a single scan. However, as the temporal
resolution of these single scans was insufficient to observe the rapid
variation of the blob, we developed high-speed meridional scans for this
study, with a temporal resolution that is ∼ 25 times higher
than that of the CP-3 scans. A detailed description of the CP-3 scans was
introduced in an earlier publication (see Ishida et al., 2014, Fig. 1).
Since the EISCAT observations cannot capture the large-scale picture around
the field of view (FOV) due to its limited observational range (up to
∼ 5∘ of the latitude), we used TEC maps to visualize
the horizontal shape of the density structures. In addition, the convection
pattern around the EISCAT FOV was inferred from the ground-based
magnetometer data.
During the observation campaign, we encountered a substorm on 4 December
2013 at 17:00–18:00 UT, which was detected using the geomagnetic data. The
beginning of the negative bay was shown at ∼ 17:00 UT in the
magnetometer H-component of Dixon Island (73.54∘ N,
80.56∘ E) and Tixie Bay (71.58∘ N, 129.00∘ E)
located in the magnetic midnight sector, which means that the substorm
growth phase started at this time. Subsequently, it explosively developed at
∼ 17:30 UT; at the same time, Pi2 pulsation also appeared at
Nurmijärvi (56.89∘ N, 102.18∘ E). Moreover, the
auroral electrojet (AE) index suddenly increased at ∼ 17:35 UT. Thus, we concluded that the onset time was around 17:30 UT. The
interplanetary magnetic field (IMF) was predominantly southward (-4.4 to
-3.4 nT) during 17:00–18:00 UT. In addition, the IMF By
component turned from positive to negative (down to ∼-2.0 nT)
at ∼ 17:40 UT.
Results
Figure 1 shows the TEC maps after onset of the substorm. The color scale
runs from a high-electron density (red) to a low-electron density (blue).
The top left panel of Fig. 1 is an overview of the polar ionosphere around
onset time (∼ 17:30 UT). The dashed black line represents the
solar terminator, where SZA = 90∘. Besides, the top right panel
is the TEC variation around the EISCAT FOV, which enlarges the black box
shown in the top left panel. The black rectangle indicates the EISCAT FOV,
and the dashed curving line indicates the boundary of the high-latitude
trough. The earlier study used a threshold of just 20 % lower than the
background ionospheric electron density to identify the ionospheric trough
(Ishida et al., 2014). In this event, therefore, the region of less than
∼ 4.8 TECU (total electron content unit; 1 TECU = 1016 el m-2) can be considered
as the trough region since the background ionospheric electron density was
∼ 6.0 TECU in the nightside. Thus, we can see that the EISCAT
FOV was inside of the trough region after onset of the substorm. Figure
1a–f show a time sequence of the horizontal shape of the density
structures around the EISCAT FOV. Note that the color scale ranges within
0.5–3.5 TECU so that the focus is on the density variation in the trough
region. A sequence of structural change shown in Fig. 1a–c illustrates that
a chunk of increased plasma density gets closer to the EISCAT radar during
17:30–17:45 UT, and thus the TEC value slightly increases around the
high-latitude side of the EISCAT FOV (up to ∼ 3 TECU in Fig. 1c). Then, the horizontal shape changes intricately around the EISCAT FOV
during 17:45–18:00 UT (d–f). As described above, the TEC maps indicate
that the EISCAT radar observed the plasma structuring in the pre-midnight to
dusk subauroral region, and the structuring was shown in the trough region.
Thus, the blob deformation, which is shown in detail later, likely can be
regarded as that of a subauroral blob.
Observations from EISCAT and the IMAGE magnetometer during the
substorm on 4 December 2013. The keogram was reproduced from the
Ne observed by the meridional scans at an altitude of 210 km,
with overplotted convection vectors at an altitude of 120 km from the IMAGE
meridian chain. The red dashed vertical line indicates the onset time
(∼ 17:30 UT), and the remaining 30 min is divided into six
sections according to Fig. 1a–f.
Figure 2 shows the keogram at an altitude of 210 km that was reproduced from
the Ne observed by the meridional scans, with overplotted
convection vectors. A keogram is the time versus geomagnetic latitude plot
of the EISCAT plasma parameters along a specific cross section. In this
study, we show a keogram along the geomagnetic north–south cross section.
The convection vectors were from the International Monitor for Auroral
Geomagnetic Effects (IMAGE) meridian chain (see station information at
http://space.fmi.fi/image/beta/?page=maps). Note that the convection
vectors are in the opposite direction from the equivalent current vectors
derived from the IMAGE magnetometer data and the vector size does not
reflect the actual drift speed but the geomagnetic variation. The red dashed
vertical line indicates the onset time of the substorm (∼ 17:30 UT), and the remaining 30 min is divided into six sections
according to Fig. 1a–f. A high-density blob appears at ∼ 17:20 UT, and then it extends toward the low-latitude region by the end of
the observations (18:00 UT). The overplotted vectors shown in Fig. 2
indicate that the southeastward flow was dominant above ∼ 71.5∘ N, while the southwestward flow was dominant below
∼ 68.5∘ N during 17:00–17:40 UT. Therefore, we can
say that a southeast–southwest flow shear existed between 68.5∘ N and 71.5∘ N at least by ∼ 17:40 UT. Since the
southeastward flow region expanded equatorward from ∼ 17:20 UT
to 18:00 UT, the latitude of the flow shear seemed to move over time, and it
appeared to be located at a lower latitude than the blob region after
∼ 17:50 UT. In addition, it should be noted that the
southeastward flow increased rapidly at ∼ 68∘ N
after ∼ 17:50 UT.
The temporal evolution of a blob on the meridional plane during
Fig. 1e–f, with geographic latitudes on the horizontal axes and altitudes
on the vertical axes: (top) Ne, (middle) Vi(N-S), and (bottom) Ti. Dashed slant lines
indicate the geomagnetic field lines. Red horizontal axes indicate the
geomagnetic latitudes at an altitude of 210 km.
Figure 3 shows the temporal evolution of a blob on the meridional plane
during Fig. 1e–f: (top) Ne, (middle) north–south component
of ion velocity (Vi(N-S)), and (bottom) Ti. Note that the Vi(N-S) was calculated from
the Vi(LOS) and the elevation angle of the meridional scan.
Dashed slant lines indicate the geomagnetic field lines. Red horizontal axes
indicate the geomagnetic latitudes at an altitude of 210 km. Hereafter,
unless otherwise noted, the geomagnetic latitude refers to that in Fig. 3.
Since the Vi(LOS) and Ti deduced from
incoherent scatter fitting are usually inappropriate in the case of
Ne<5×1010m-3, we ignored them in
such situations. Figure 3a shows that a part of the blob appears at the
high-latitude side of the EISCAT FOV, which was nearly superposed on the
region of southward velocity. Figure 3b–c show that the blob seems to be
cracked around 68.5∘ N (red arrow in Fig. 3b) at ∼ 17:52 UT, and then it is clearly divided into two parts (Blob A and Blob B)
over the boundary where the Vi(N-S) reversed from northward
to southward (hereafter referred to as the Vi(N-S) reversal
boundary), whose northward component reaches to ∼ 400–500 m s-1. The scale sizes of the divided blobs were ∼ 100 km in
latitude (Blob A) and more than 250 km in latitude (Blob B). It can also be
seen that the Vi(N-S) reversal boundary was roughly along
the geomagnetic field line. Blob A was distributed at altitudes between 180
and 300 km, while blob B was distributed at even lower altitudes.
Furthermore, the cross section between the two blobs was roughly along the
geomagnetic field line. Figure 3d shows that the divided blobs appear to be
elongated equatorward, and the velocity difference on the Vi(N-S) reversal boundary reached to ∼ 800 m s-1. Another
feature identified in Fig. 3 is that the Ti partially
increased around the location where the blob started to be divided in Fig. 3b, and the values reached up to ∼ 1400 K.
The horizontal distribution of the convection vectors at
∼ 17:55 UT. The green, red, and black vectors are the
convection vectors from the IMAGE magnetometer. The large red arrow
indicates the localized northwestward flow, which was assumed from the
horizontal distribution of the convection vector and EISCAT data. The line
plot shows the latitudinal variation of the Vi(N-S) at
∼ 17:55 UT.
Discussion
In this section, we evaluate the physical and chemical processes associated
with the blob deformation shown in Fig. 3. Figure 4 shows the horizontal
distribution of the convection vectors at ∼ 17:55 UT (the time
when the blob deformation was observed). The green, red, and black vectors
are the convection vectors from the IMAGE magnetometer. The southeastward
flow (green vectors) was located at the east side of the EISCAT radar, while
the southwestward flow (red vectors) was located at the low-latitude side.
From the above circumstances, we presumed that southeastward–northwestward
flow shear was located around the EISCAT observational region. As mentioned
in Sect. 3, the Vi(N-S) was southward at the
high-latitude side of the FOV, while it was northward at the low-latitude
side of the FOV at ∼ 17:55 UT. Thus, we assumed here that
EISCAT observed the localized northwestward flow (a large red arrow in Fig. 4) accompanied by southeastward–northwestward flow shear. It is known that
the plasma flow shear is enhanced in the pre-midnight to dusk region during
the substorm expansion phase (e.g., Iijima and Nagata, 1972). Such plasma
flow shear is considered to be formed by a combination of the DP 2 field
(convection current system) and DP 1 field (substorm current system) (see
Iijima and Nagata, 1972, Fig. 9). Moreover, a statistical analysis using
SuperDARN data also indicated that the zonal convection speed is enhanced in
the pre-midnight to dusk region during the substorm expansion phase in the
case of IMF By<0 (see Grocott et al., 2010, Fig. 4d). Hence, if
EISCAT actually observed the enhanced plasma flow shear accompanied by the
substorm expansion phase, the blob deformation might have been caused by the
enhanced flow shear through the Kelvin–Helmholtz instability (KHI). Thus,
we investigated the possibility that the blob deformation was caused by the
enhanced flow shear through the KHI. To evaluate the blob deformation in a
quantitative way, we calculated the linear growth rate of the KHI on the
basis of some assumptions. Early studies used 0.2 V/L as
the linear growth rate of the KHI, and thus the growth time τKH
turns out to be 5 L/V, which is the inverse of the growth rate
(Carlson et al., 2007). Here, V is the velocity difference and L is the
scale length of the velocity difference. As shown in the line plot of Fig. 4, the Vi(N-S) reverses sharply from
∼ 450 m s-1 (northward) to ∼-300 m s-1 (southward) between
68 and 69∘ N along the geomagnetic field line. Assuming
that the velocity difference on the Vi(N-S) reversal
boundary is derived from the zonal (southeastward–northwestward) flow
shear, we can find that the scale length L is ∼ 0.8∘ of the latitude (see dashed red rectangle in Fig. 4), which
corresponds to ∼ 90 km at an altitude of 210 km. Considering
that the blob starts to deform at ∼ 17:52 UT, the KHI would
have grown during ∼ 3–4 min (∼ 3.5 min on
average) by the time of Fig. 3c. Thus, we can find that the velocity
difference of the convection shear Vi(shear) has to be
larger than ∼ 2100 m s-1 by the equation τKH≤3.5 min⇒V≥5×90 km/3.5 min. Plasma
flows of thousands of meters per second are often observed during substorms
(e.g., Zesta et al., 2011). If such zonal (southeastward–northwestward)
flow shear really existed, the angle θ between the shear plane and
meridional plane must be larger than ∼ 70∘ according
to the equation Vi(shear) = Vi(N-S)/cos(θ)≥2100 [m s-1], with Vi(N-S)∼ 750 [m s-1] around the shear region.
Besides, Fig. 4 indicates that the angle θ is seemingly larger
than ∼ 70∘ around the EISCAT FOV; in addition,
Figure 2 also shows the convection vectors are inclined larger than
∼ 70∘ from the meridional plane after
∼ 17:52 UT around 68∘ N, which also support the
above assumption. Hence, it is possible that the KHI could play a role in
modulating the blob structure under the above assumption, although this is
not a conclusive answer for the blob deformation.
Next, we discuss the extent to which chemical processes could have
influenced the blob deformation. As mentioned in Sect. 3, it was found
that the Ti partially increased around the location where the
blob started to be divided in Fig. 3b. In such a case, there is the
possibility that frictional heating will cause blob cutting if the
dissociative recombination promotion is fast enough at the localized heated
region. An early study indicated that the tongue of ionization (TOI) was
divided into two patches by high-speed plasma jets in excess of 2 km s-1
through the enhanced recombination process (Valladares et al., 1994). Since
we could identify that the blob deformed at ∼ 150–300 km
altitudes from Fig. 3b–c, we compared the recombination rates by altitude
to evaluate whether or not dissociative recombination could have promoted
this on a timescale of minutes. The recombination rate β used here
was estimated from the mass spectrometer incoherent scatter (MSIS) model
parameters and theoretical equations introduced by St.-Maurice and Torr
(1978). Note that for simplicity, we used a recombination rate for/at
Ti = ∼ 1400 K at all altitudes. Then, we can
confirm that β is nearly equal to ∼ 310 × 10-4 s-1 at 150 km, ∼ 58 × 10-4 s-1
at 180 km, ∼ 16 × 10-4 s-1 at 210 km, and ∼ 5 × 10-4 s-1 at 240 km, which correspond to recombination times of
∼ 0.5 min, ∼ 3 min, ∼ 10 min, and
∼ 33 min, respectively. Considering that the blob starts to
deform at ∼ 17:52 UT, the dissociative recombination process
would have been promoted for ∼ 3–4 min by the time of Fig. 3c. Hence, the results indicate that there would have been sufficient
recombination time to allow for dissociative recombination and the blob
cutting scenario at roughly less than 180 km of altitude, but it is
difficult to explain that at higher altitudes.
Conclusions
To the best of our knowledge, this is the first detailed report containing
direct observations of blob deformation at the nightside auroral region
during a substorm, and our report is accompanied by an evaluation of the
plausible processes that could have contributed to blob deformation. During
the substorm expansion phase, the blob seems to have divided into two parts,
whose scale sizes were more than ∼ 100 km of the latitude. We
then discussed the possible physical and chemical processes that may have
caused this blob deformation. Our analysis indicated that the KHI and
dissociative recombination could have influenced the blob structure.
The Supplement related to this article is available online at doi:10.5194/angeocom-33-525-2015-supplement.
Acknowledgements
This research was financially supported by the Japan Society for the
Promotion of Science (JSPS) Fellows program and the Center for the Promotion
of Integrated Sciences (CPIS) of SOKENDAI. We are indebted to the director
and staff of EISCAT for providing the observational data. EISCAT is an
international association supported by research organizations in China
(CRIRP), France (CNRS, until the end of 2006), Finland (SA), Germany (DFG,
until the end of 2011), Japan (NIPR and STEL), Norway (NFR), Sweden (VR),
and the United Kingdom (NERC). Ingemar Häggström is also
acknowledged for his helpful support in operating the EISCAT peer-reviewed
program experiments and special experiments in Tromsø.
The authors are thankful for the GPS-TEC data that were collected under the
direction of A. J. Coster at the MIT Haystack Observatory. The TEC map was
obtained from the GPS/TEC Plot routine on the Virginia Tech SuperDARN
website (http://vt.superdarn.org/tiki-index.php?page=DaViT+TEC), which
is supported by NSF award numbers AGS-0838219 and AGS-0946900.
The IMAGE magnetometer network is maintained by 10 institutes from Estonia,
Finland, Germany, Norway, Poland, Russia, and Sweden. The authors are
grateful for their continuous effort in maintaining the receivers. The topical editor Steve Milan thanks one anonymous referee for help in evaluating this paper.
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