ANGEOAnnales GeophysicaeANGEOAnn. Geophys.1432-0576Copernicus PublicationsGöttingen, Germany10.5194/angeo-34-165-2016Generation of a severe convective ionospheric storm under stable Rayleigh–Taylor conditions: triggering by meteors?KelleyM. C.mck13@cornell.eduIlmaR. R.School of Electrical and Computer Engineering, Cornell University, Ithaca, NY 14853, USAM. C. Kelley (mck13@cornell.edu)3February201634216517025September201527December20155January2016This 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/34/165/2016/angeo-34-165-2016.htmlThe full text article is available as a PDF file from https://angeo.copernicus.org/articles/34/165/2016/angeo-34-165-2016.pdf
Here we report on four events detected using the Jicamarca Radio Observatory (JRO)
over an 18-year period, in which huge convective ionospheric
storms (CISs) occur in a stable ionosphere. We argue that these rare events
could be initiated by meteor-induced electric fields. The meteor-induced
electric fields map to the bottomside of the F region, causing radar echoes
and a localized CIS. If and when a localized disturbance reaches 500 km, we
argue that it becomes two-dimensionally turbulent and cascades structure to
both large and small scales. This leads to long-lasting structure and, almost
certainly, to scintillations over a huge range of latitudes some ±15∘ wide and to 3 m irregularities, which backscatter the VHF radar
waves. These structures located at high altitudes are supported by vortices
shed by the upwelling bubble in a vortex street.
Ionosphere (equatorial ionosphere; ionospheric irregularities) – radio science (ionospheric physics)Introduction
Many features of convective ionospheric storms (CISs), a.k.a. equatorial
spread F (ESF), are understood now after 90 years of studies (see review by
). A CIS manifests itself as vertically elongated wedges of
depleted plasma (bubbles) that drift upward from beneath the bottomside F region to the plasmasphere. Most of the CIS-related phenomena have been
related to the ionospheric interchange instability .
Here we examine data, some of them collected almost 20 years ago, that are so
baffling they have not even been published. The CIS events presented in this
study occurred in the bottomside of the F region with a very low growth rate.
These rare events rapidly develop into a narrow plume reaching F region
topside altitudes. We provide a possible explanation associated with the
seeding of ESF by meteor-induced electric fields. Such fields have been
invoked to explain long-duration radar echoes, attributing them to the Farley E region
instability driven by these fields. The Farley
instability is excited if the component of the electron–ion relative drift in
the direction of propagation of the irregularities
(k⋅VD)
exceeds the ion acoustic speed (Cs). Extensive details on CISs and Farley
instability can be found in . There is ample
evidence that electric fields driven by meteors exist, and here we argue that
they may also induce CISs in the F region.
Data Presentation
Figures and show the two initial examples we
found many years ago but never understood. According to the World Data Center (WDC)
for Geomagnetism, Kyoto, the magnetic activity during these two events
was low (Kp = 2). More typical events are presented in
. The dark areas show regions of backscatter from
3 m irregularities detected by the large Jicamarca radar in Peru (11.95∘ S, 76.87∘ W; 0.6∘ N dip latitude, 1.0∘, dip
angle). The system works like a slit camera looking straight up and the
resulting view looks south at the ionospheric equatorial plane with east to
the left and west to the right (see Fig. ). For a rigid
structure moving at the typical value of 100 m s-1, the scales are designed
such that the shape resembles a real photo.
Range–time–intensity plot of spread-F irregularities on 8
September 1993. The C-shaped structure appears at 22:45 LT. The level of magnetic activity during the event is low (Kp = 2).
The same as Fig. 1 but on 9 September 1993. This event occurred during low magnetic activity (Kp = 2).
Vertical velocity of plasma irregularities corresponding to Fig. 1.
Cartoon showing how a slit camera would view the plume from a boat moving east into a velocity shear.
Eighteen range–time–intensity plots of spread-F events. Using the JULIA radar, these are similar-looking events.
These data have many unusual aspects. When these impressive plumes erupt, the
ionosphere is very low and descending in both cases, meaning that the
equatorial zone is nearly stable, or at least has a very low growth rate, for
CISs driven locally. In Fig. , essentially no initial
irregularities exist prior to the eruption, that is, no local seeding is
present. The gray areas in the plot most likely are noise. The initial size
of the plumes, corresponding to their 5 min duration and to a spatial
structure drifting eastward at 100 m s-1, is 30 km. The final state in the two
cases is similar in some ways and very different in others.
In Fig. , the narrow structure rises straight up and expands
somewhat above 500 km and then seems to explode to 900 km, the top of the
data limit. Notice that, although there is turbulence in the plume, it is
less extensive than in Fig. . called
the structure in Fig. a C-shaped structure. There are at least
two explanations for the C-shaped feature. argued
that, as a bubble or density depletion rising in the presence of a downward
background electric field, it polarizes such that the internal electric field
is upward. This means the bubble plasma moves slower than the background and,
since it is turbulent, the radar signal follows it. Thus, the plume signal
comes to the radar beam as a function of height forming the top of the C. The
bottom connects to the bottomside layer to finish the C. The second
explanation is that the downward electric field component is partially
shorted out by the off-equatorial E region . The
bottom portion of the C is completed by a westward flow in the region below
the F peak in either case. Figure
exhibits a small C, almost no bottomside echoes, and the initial plume width
is even smaller than in Fig. . However, above 500 km, it erupts into
a huge structure with a turbulent volume of over 4 × 108 cubic
kilometers, where we have used the length of the magnetic field line for the
third dimension. We only have line-of-sight Doppler information on 8 September, as shown in Fig. .
Some of the echoes in Fig. are from the irregularities, not
from incoherent scatter, but the drifts before and after the plume agree with
the motion of the layer. Every aspect of the event indicates that the
generalized Rayleigh–Taylor instability (GRTI) was stable before the events.
The electric field was westward, so the E×B instability
was stable on the bottomside. Due to the low altitude, the GRTI growth rate
was small. The Doppler velocity agrees with the motion of the echoing region every time and indicates that a large perturbation electric field (1 mV m-1)
developed quickly at 22:40 LT. We cannot determine the solar wind electric
field at this time since there was no upstream satellite, but a temporal
pulse of penetrating electric field, rather than an unstable (preexisting)
feature, would not have time to create the plume. For example, at 50 m s-1 the
plume would take over 1 h to rise to 450 km, not less than 5 min as
indicated. Thus, the structure must have existed prior to 22:40 LT and just
drifted over the site with its internal perturbation field inside. This
reminds us of a snapshot of a boat, with its smokestack billowing, traveling
to the left with the plume entering a region of velocity shear, as shown in
Fig. .
Two of the Fig. 5 events are shown, which we believe are similar to those in Figs. 1 and 2. These events occurred under moderate magnetic activity. The Kp indices are 3 (top) and 5 (bottom).
Eighteen more or less similar events are shown in Fig. . The two
events that we feel are very similar to those in Figs.
and are reproduced in Fig. . Our criterion of
similarity is based on their low initial altitude and the lack of any uplift
prior to the event. The data selected was obtained with the JULIA radar
(Jicamarca Unattended Long-term investigation of the Ionosphere and
Atmosphere) . Note that the
JULIA system transmits at a 10 db smaller power and hence the plumes in
Figs. and would look smaller to JULIA. Other possibilities we have rejected are based on the plumes' size at the base, the
launching of an “apogee” plume if the layer reaches a high enough
altitude, or plumes clearly seeded by either a gravity wave or a shear
instability. The fact that we only found four examples between 1993 and 2007 proves that these events are indeed rare.
Discussion
The events are so rare that the source must also be very rare. The source
must create a large electric field at 200–250 km and be localized in space
and time. Our only candidate is a large meteor.
We know that meteors do create electric fields; the Jicamarca radar has
detected two-stream waves due
to meteors. The cause is an ambipolar electric field due to the localized
trail of atoms and plasma as the meteor ablates, vaporizes, and ionizes
.
have presented a new model for the electrodynamics of
meteor entry into the mesosphere–lower thermosphere and the D region of the ionosphere. Their purpose
was to explain how humans are able to hear meteors at the same time they see
them. Using the geometry below they suggested that the current in the E region
launches an audio frequency radio wave, which couples on metallic objects to
a sound wave by the process of electrophonics. The idea is illustrated here
in Fig. .
In meteor radar studies there is a phenomenon known as a head echo in which a
dense plasma, as high as 1016 m-3, is formed and follows the meteor
itself. Curiously this plasma disappears a few meters behind the meteor and
is hence called a coma. Since there are known to be electrons around the meteor
head due to the radar head echo, there must also be ions. The ions are borne
with the same velocity as the meteor and must travel with it in the D region
where they are not magnetized. However, the electrons are magnetized and
cannot follow any component of the ion motion across the magnetic field,
B, unless an electric field builds up to drag the electrons along.
To accomplish this requires a large electric field since the electrons are
magnetized. To keep ni=ne, an electric field builds up such that the
ion current equals the electron Pedersen current (the component parallel to
E in the anisotropic ionospheric current). Using the electron
conductivity in the D region ,
Ex=enVmσeP,
where Vm is the meteor velocity, n is the initial ambient plasma
density, and σeP is the electron Pedersen conductivity, which is
proportional to the density. Substituting yields Ex= 10 V m-1. As discussed
by , this is greater than
the critical velocity for neutral atmospheric discharge that is known to
occur over thunderstorms, creating the phenomenon of sprites. We argue that
the E field saturates at the critical value for discharge, which, at the
meteor ablation height, is about 2 V m-1.
The region of discharge is behind the meteor and only as large as the size of
the electric field. We estimate this as follows. The separation of ions and
electrons can only be on the order of a few Debye lengths (λD). In the
initial background ionosphere the Debye length is λD=Ve/fp, where
Ve is the electron thermal speed (about 106 m s-1) and fp is the
plasma frequency (about 105 Hz). Thus, initially λD is about
10 m and the electric field extends 20–30 m. The physics after this time is very
complex as the electrons heat and the plasma density increases. The two
effects tend to cancel in the estimate of the electric field length.
Geometry of the model. Here we take the optimum case for a meteor velocity perpendicular
to B. The electric field generated opposite of the meteor velocity maps up to the E region since
the conductivity along B is very high and is the source of the electromagnetic radiation needed for electrophonics.
The Em field is the component we discuss below and which travels to the bottom of the F layer as a whistler pulse.
This electric field is launched as a millisecond-long whistler mode wave pulse along
the field line, arriving at the base of the F region almost unattenuated.
Within 1 s the entire length of the meteor track (10–40 km) has launched
this large field, which when it reaches the base of the F layer lifts the
plasma at a huge velocity, some 80 km s-1. This clearly punches through to the
topside, creating a huge disturbance,ff7
and certainly can cause the observed
phenomena.
showed that a 30 km bubble will begin to
behave in a two-dimensionally turbulent manner at
a height of about 500 km. When this occurs, the turbulent region will undergo
an inverse cascade to larger and larger sizes, explaining the huge volume of
radar scatter in these events. The huge path lengths also increase the
scintillation of radio waves. An event like the one in Fig. will
cause an S4 of nearly 1 at GPS frequencies.
The transition between the collisional and collisionless Rayleigh–Taylor
instability occurs at a height where the ion-neutral collision frequency is
such that
νin=16UR,
where R is the two-dimensional bubble radius and U is the bubble velocity
. In this case, the growth rate is
γ=gΩiL,
where Ωi is the ion gyro frequency and g is the gravitational
constant. Initially in the collisionless case, the wedge (bubble) accelerates
at -g. The terminal velocity in this case generates waves at marginal
stability. Since this is all occurring perpendicular to B, the
instability is the Farley two stream, in which the ion current, driven by
gravity, exceeds the sound speed, Cs. In the resulting steady state, the
charge that tends to build up inside the bubble as it tries to accelerate is
shed back into the medium by vortices that occur in a vortex street behind
the bubble . A vortex in a two-dimensional plasma
is equivalent to a line charge, positive east of the bubble and negative west
of it. Evidence for such vortices can be seen in Fig. . The
terminal velocity is thus about 1 km s-1, a typical upwelling velocity at high
altitudes . The vortices must be long-lasting to
support 3 m echoes for hours. This can only be maintained by the forward cascade
of enstrophy down to meter scales, as occurs in two-dimensional turbulence
. Furthermore, such
circulation in a high-density plasma will tend to put the neutral atmosphere
in a motion similar to motions in the high-latitude polar circulation zone
. In this case, with a plasma density of 106 cm-3,
we predict that a neutral vortex will form in 0.5 h. However,
such features would be very difficult to detect using current techniques.
Conclusions
We believe the unusual events reported here are due to large meteors
impacting the E region off the equator. The resulting electric fields map to
the bottomside of the F region, causing radar echoes and a localized
convective ionospheric storm. Once the localized disturbance reaches 500 km,
it becomes two-dimensionally turbulent and cascades structure to both large
and small scales, leading to long-lasting scintillation over a huge range of
latitudes some ±15∘ wide and to 3 m irregularities that
backscatter the VHF radar waves. At high altitudes, the steady state is
supported through vortices shed by the upwelling bubble in a vortex street.
Meteor events have led to simulations of the Farley instability. This study
might encourage meteor simulations of F region instabilities in the
future.
The Supplement related to this article is available online at doi:10.5194/angeo-34-165-2016-supplement.
Acknowledgements
The work at Cornell was supported by the Atmospheric Science Section of the
NSF and the School of Electrical and Computer Engineering at Cornell.
The topical editor, H. Kil, thanks two anonymous referees for help in evaluating this paper.
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