Bright comets display a rich variety of interesting plasma phenomena which occur over an enormous range of spatial scales, and which require different observational techniques to be studied effectively. Wide-angle photography of high time resolution is probably the best method of studying the phenomenon of largest known scale: the plasma tail disconnection event (DE), which has been attributed to magnetic reconnection at interplanetary sector boundary crossings. These structures usually accelerate as they recede from the head region and observed velocities are typically in the range 50  V  km/s. They are often visible for several days following the time of disconnection, and are sometimes seen out past 0.2 AU from the cometary head. The following areas pertaining to plasma phenomena in the ionoshere are addressed: the existence, size, and heliocentric distance variations of the contact surface, and the observational signatures of magnetic reconnection at sector boundary crossings

Niedner, M. B., Jr.

English

NASA Technical Reports Server

OBSERVATIONAL DATA NEEDS FOR
PLASMA PHENOMENA
Malcolm B. Niedner, Jr.
Laboratory for Astronomy and Solar Physics
NASA/Goddard Space Flight Center
Greenbelt, MD 20771
Abstract
Bright comets display a rich variety of interesting plasma phenomena which occur over an
enormous range of spatial scales, and which require different observational techniques to be
studied effectively. Wide-angle photography of high time resolution is probably the best
method of studying the phenomenon of largest known scale: the plasma tail disconnection event
(DE), which nas been attributed to magnetic reconnection at interplanetary sector boundary
crossings. These structures usually accelerate as they recede from the head region and
observed velocities are typically in the range 50 < V < 100 km s~^. They are often visible
for several days following the time of disconnection, and are sometimes seen out past 0.2 AU
from the cometary head. The recession velocity and repulsive acceleration are observable
parameters which could be incorporated into theoretical models to estimate, for example, the
plasma tail's magnetic field and mass density. The primary observational requirement is that
enough photographs to be taken with which to assemble an accurate kinematical description.
For all but circumpolar comets, this really requires a network of observatories.
Helical formations, "condensations," arcade loops, "kinks", and folding tail rays, are
smaller scale plasma tail features which tend to accompany DEs, and whose morphology and
kinematics can also be studied photographically. Possible origins of these features will be
discussed, j
i
The foljlowing areas pertaining to plasma phenomena in the ionosphere will also be
addressed: the existence, size, and heliocentric distance variations of the contact surface,
and the observational signatures of magnetic reconnection at sector boundary crossings.
I
I
It would be difficult indeed to overestimate the important and unique role of comets in the
study of cosmic plasmas. It is also difficult to summarize this topic in 10 minutes, however, and
so the first order of business is to disclaim that this talk is comprehensive.
Since cometary plasma is created in the head and then flows down the plasma tail, I would like
to follow that route in the discussion, pointing out along the way a few of those problems which I
feel are of particular importance.
The contact surface proposed to stand off the solar wind has been the subject of some recent
and interesting jwork by Delsemme and Combi. Briefly, using calibrated slit spectrograms, they
constructed ion Brightness profiles along the Sun-comet line for comets Bennett 1969i (Delsemme
and Combi 1979) land West 1975n (Combi and Delsemme 1980). Figure 1 shows some of the results for
H20+ in comet Be'nnett. The diagram displays the brightness profiles for 2 spectra taken on
the same night, J20m apart. On the sunward side the brightness falloff is rather smooth until a
distance of a few times 10^ km is reached, beyond which the profile is turbulent and wavy. In
the tail ward profiles, wavy features were seen in each spectrum which appeared to have
counterparts in ithe other spectrum. By lining up the crests and troughs as shown in Figure 2,
they deduced that the speed of these features was 17 km s-i in the tailward direction. In
Delsemme and Combi's model, the waves are bulk motions first induced on the sunward side when
cometary plasma crosses the contact surface and hits the turbulent magnetosheath plasma and
embedded magnetic field.
Using C0+ profiles in comet West, Combi and Delsemme estimated the position of the contact
surface and were even able to deduce a heliocentric distance variation of its size over the range
0.44 < r < 0.84 AU.
21
https://ntrs.nasa.gov/search.jsp?R=19820006119 2020-03-21T10:13:03+00:00Z
LUCO
</»
=
3
CUo
0
 
-
-
-
-
Ao
\
\
coo>
if)
•4-J
+
->
CUE
 
-O
E
 (O
CU
 
CU
 
•
CQ
 JE
 
-OCU
-!->
 
CU
 S
-
CU
 
-E
 
3
o
 
o
O
 
E
 
-r
-
O
 Q
-
o
 
-o
 cu
M
-
 
C
U
 i
-
S
-
 
fO
^
~
.
 0)
CTl
 (D
 (O
i
—
 
O
 
Q
-
(O
JD
 4->
 O
£
 
O
 C
M
O
 CU
o
 
a
.
 c
(/>
 
O)
•o
 
-^
E
 
+
-)
 ra
d)
 (/I
 
I/)
E
 
C
D
O)
 
OJ
CO
 
-(->
i
 —
 (O
CD
O
S
-
 
S
-
3
 
Q
.
>
,
 
O
.a
 
<
o
 3
o
 i
—
-oCD
 
E
E
 
O
 
•
•r
-
 
S
-
 O)
J2
 T3
 i
—
O
 CU
to
 
-i
-
 C
U
CU
 
n3
 
E
i
—
 
+
->
 O
•i
-
 
-Q
 O
M
-
 
O
 
I
O
 
C
S
-
 
C
U
 3
C
L
 S
-
 00
to
 
S
 C
U
to
 
J
=
cu
 
to
 
-i->
E
 C
U
•!->
 r
-
 C
D
J
=
 
-r
-
 
E
D
IM
-
 O
•r
-
 
O
 •
—
S
-
 
S
-
 
rO
O
 
O
-o
+
 
cu
 cu
O
 
-E
 4->
CM
—
 E
:n
 
cu•r—
•
 
s
-
•
 
-r
-
 
O
r
—
 
O
1
CU
 O
1
 
E
S
-
 r
-
 (O
CNISS3N1H9IN8
 3AI1V13H
 90122
CO
001 2
b
B 0.5 1.0 1.5
PROJECTED DISTANCE (105 kilometers)
Figure 2. The antisunward profiles of Figure 1 have been
shifted 18,000 km relative to one another, bringing
the crests and troughs visible in each profile into
coincidence. The inferred speed of these features
down the tail is 17 km s~^ (from Delsemme and Combi
1979).
23
To my knowledge, observations of this type are unique to these 2 studies and I would
personally like to see a lot more of this work in the future. Assuming that this is a valid
technique for detecting the contact surface, important questions that could be addressed in the
future are these:
1) Does the solar wind penetrate down to the nucleus of faint comets with low gas
production rates, or is it stoo'd-off by a contact surface.
2) Is the observed disappearance of the plasma tail with increasing heliocentric distance a
result of sub-detection brightness levels, or of the destruction of the contact surface.
3) What correlation, if any, exists between local solar-wind conditions and the size of the
contact surface.
For the rest of the talk, I would like to discuss a subject on which Jack Brandt and I have
collaborated for the last few years: plasma tail disconnection events, or "DEs" (Niedner and
Brandt 1978, 1979, 1980). Two reasons why I think DEs are so fascinating are, first, they
constitute the only really predictable response of a plasma tail to a particular feature in the
solar wind, and second, so much more goes on than just the tail coming off. In fact, the
disconnection process may provide a unification of many disparate forms of plasma tail activity,
as is suggested in Table 1. Note that activity in the head, as well as in the tail, is thought to
be related to the DE phenomenon. I will try to justify these connections shortly, but first, I
would like to present a quick review of our model of DEs.
Figure 3 shows the arrival of a sector boundary to a comet (Niedner and Brandt 1978).
Starting in panel B, after the sector boundary has just swept past, reconnection of field lines in
the head progressively severs the plasma tail fields which had been frozen-in. About 18 hours
later, the tail has become totally disconnected (panel C). The capture of flux from the new
magnetic sector then starts building a new tail, and the entire process repeats when the next
sector boundary comes along. Thus, a bright comet which is observed for several weeks might
experience several events. I am personally aware of 5 DEs in Hal ley's Comet during the 1910
apparition, and of 9 events in comet Morehouse in 1908. I would be very remiss if I did not point
out that Ip and Mendis (1978) have propsed a different mechanism for DEs which involves high-speed
streams. We still prefer the sector boundary model (Niedner and Brandt 1979), however, and I will
stick with it in the following discussion.
The reconnection phase of a DE (panel B in Figure 3) could produce noticeable effects in the
head as a result of the dissipation of magnetic energy, namely, an ionization surge and brightness
increase (Niedner 1980). There is observational evidence that this actually happens. Figure 4 is
a qualitative light curve constructed from Barnard's descriptions of naked eye observations of
comet Morehouse; he was convinced that two major brightness flares occurred, one on about
October 14, the other on October 29. Both of these brightenings correlate beautifully with DEs,
and Figure 5 shows the first one in a photograph taken at Indiana University on October 15. Such
associations between brightness flares and tail disconnections are very exciting and convincing,
but they will actually be useful to theoretical reconnection models only when we have observed
absolute flux increases during the time of a DE. Narrow-band photometry centered on the
wavelengths of ion emissions would be particularly useful.
If reconnection is indeed the mechanism of DEs, then it would also be extremely worthwhile to
look for ion radial velocities in the head. Figure 6 shows the rationale for this assertion.
Heated plasma flows out of both sides of the reconnection region (the hatched rectangle) at the
Alfven speed, which is probably about 5 km s~l, but might be as high as 10 km s-1. Probably
more promising is the jetting of material perpendicular to the plane of the figure in response to
the curl of the magnetic field in the field reversal region. These speeds may be significantly
higher and would be worth looking for. Also, filtered photographs of short exposure which isolate
particular ions might actually show some of these jets and reconnected flux tubes.
Figure 7 is a schematic of 4 phases in the life of a plasma tail which qualitatively
illustrates the interrelationships mentioned earlier in Table 1. In Phase I, the main tail is
clumpy and full of "condensations," and a widely-inclined ray pair (or pairs) is also present.
Time sequences of comets in this phase sometimes show that the inner part of the plasma tail is
narrowing down. Physically, we interpret this phase as the final phase of reconnection after a
sector boundary has been traversed, but before the tail has actually come off. The tail rays are
flux tubes composed of magnetic fields captured from the new sector; the narrowing is caused by
24
Table 1
THE MANY MANIFESTATIONS
OF A DISCONNECTION EVENT (DE)
REGION FEATURES
HEAD
PLASMA TAIL
IONIZATION SURGE AND BRIGHTNESS FLARE PLASMA JETTING
NARROWING DOWN OF INNERMOST TAIL PRIOR TO DISCONNECTION
"CONDENSATIONS" BEFORE AND AFTER DISCONNECTION
ENHANCED TAIL BRIGHTNESS IMMEDIATELY BEFORE DISCONNECTION
PROMINENT TAIL RAY ACTIVITY
DISCONNECTED TAIL
HELICES AND WAVES IN DISCONNECTED TAIL (SOMETIMES).
Table 2
PLASMA TAIL STUDIES
TYPE OF OBSERVATION .OBJECTIVE
PHOTOGRAPHIC TIME SEQUENCES,
MOVIES
RADIAL VELOCITIES
BRIGHTNESS MEASUREMENTSii
IN SITU MEASUREMENTS OF
HALLEY'S COMET
MANNER OF FORMATION AND EVOLUTION OF
TAIL STRUCTURES
KINEMATICS OF TAIL STRUCTURES
(ESPECIALLY ACCELERATIONS)
PLASMA TAIL TAXONOMY AND DERIVATION OF
INTERRELATIONSHIPS
RESOLUTION OF BULK VS. WAVE MOTION
DEBATE
ION DENSITIES IN TAIL, AND VARIATIONS
BOW SHOCK, CONTACT SURFACE, MAGNETIC
FIELD STRENGTHS, CURRENTS, DENSITIES,
ETC.
25
n SOUR
4J.WIND
SECTOR
'BOUNDARY
VISIBLE PLASMA TAIL
^SECTOR
BOUNDARY
«- DISCONNECTED
PLASMA TAIL
Figure 3. The sector boundary model of plasma tail disconnection
events (DEs) (from Niedner and Brandt 1978).
26
COCXIIoo
CO
 
Ol
-
 
J
-
-o
 
s
-
i
.
 
3
<tj
 
O
C
 
(J
s
-
 
o
<OCQ
 
oj>
E
 
tJ
O
 
£
S
-
O)
 
T3
4->
 
O
»
O
 
i
-
3
 
S
-
CO
 
C
C
 
-
r
-
oO
 O)i.
CO
 (/>
0
 
-(->
en
 
co
i
—
 
S
_
3
01
 
-O
(/I
 4->
3
 
3
o
 
o
s
-
o
S3
 
-M
 *
~
QJ
 
1
.
E
 
O
o
 
••->
o
 <
o(O
(U3
 (L
I
O
 
-Mo
O
l
0
1
CM
•
—
 
CO
 
J
_
c
 <
u
O)
 
O
 JO
>
 
-I
-
 O
•i
-
 
-!->
 4->
•*->
 
<O
 U
(O
 
>
 
O
+->
 
s
-
•
-
 
cu
 
-o
r
—
 
C
O
 
C
fO
 
JD
 
fO
3
 
O
C
7
 
^
~
O)
 i
—
<c
 
>
,
O)
 
S
-
I
 (1)
(1)
 
O
O)S-CD
<O
 
(J
C
 
O
27
Figure 5. Indiana University photograph of comet Morehouse 1908c on October 15,
1908, showing a detached plasma tail. This DE is associated with the light
outburst which occurred on October 14 (see Figure 4).
28
CONTACT SURFACE
Figure 6. Schematic diagram showing the topology of magnetic fields| upstream of the contact surface at the time of a sector boundary
i crossing.
29
PLASMA TAILS: A MORPHOLOGICAL SEQUENCE
Phase I:
Narrowing tail ("Streaming")
"Condensations" in tail (Sometimes)
Strong ray system
Phase II:
Disconnection of Tail
Helical Structures in Disconnected Tail
(Sometimes)
Turning of Rays
Phase III:
Recession of Disconnected Tail
Coalescence of tail rays to form new tail
with "condensations" (Sometimes
observed)
Dynamic interaction between old and new
tails (Sometimes)
Phase IV:
Disappearance of Disconnected Tail
Diffusion of condensations
Cessation or reduction of ray activity
Return to normal appearance
Figure 7. Schematic diagram of the morphological sequence which is observed
to occur around disconnection events.
30
a.
' I H, -'. • ,
«
Figure 8. A three-hour sequence of Indiana University
photographs of comet Morehouse 1908c showing the early
recession of the end of a disconnected tail (arrows)
from the cometary head, on 1908 September 30.
31
the destruction of the old tail, and the condensations in the tail are due to the unsteady
reconnection of fields, probably a result of the tearing mode instability.
In Phase II, the tail has become disconnected, sometimes showing wavy and helical formations,
and the rays are closing to form a new tail axis and a new tail. Typical recession speeds are
50 < V < 100 km s~l. In Phase III, the new tail has been formed, but sometimes with the
appearance of arcade loops normal to the rays where they came together. This seems to be a result
of reconnection occurring between the oppositely-polarized rays. Finally, in Phase IV, the new
tail has quieted down, and the comet awaits the next sector boundary.
Observational needs for plasma tail studies are fairly obvious, and they are listed in
Table 2. Since most of the features we have been discussing are transients, photographic
sequences of high time resolution are absolutely indispensable, and I have high hopes that the
upcoming apparition of Halley's Comet will provide the organized networks required to carry out
this task. We want to know, in more detail than ever before, how features develop and evolve,
what their kinematics are, and how each class of structure fits into the grand puzzle of plasma
tail morphology and time variations, so that plasma physics models can be based on observations to
the greatest possible degree.
The time has also come for some definitive radial velocity measurements of plasma tail
structures, with particular emphasis on the bulk vs. wave motion debate. I personally feel that
both motions exist, and if I had to bet my life on one of the bulk variety, it would be the
recession of the end of a disconnected tail. This is because the ions at the very end of the tail
have an identity which doesn't change from one photograph to the next. Figure 8 is a 3-hour
sequence of comet Morehouse just after its plasma tail had come off on 1908 September 30. The
recession speed was 61 km s~s but more importantly, the component along the line of sight was a
hefty 49 km s~l. A spectrograph looking at the right place and at the right time would almost
certainly have seen an obvious Doppler shift. The chances for future success in this area would
be greatly aided by real-time monitoring of the plasma tail using an image intensifier, combined
with accurate forecasting of DEs based on the times of past events and an up-to-date knowledge of
the interplanetary sector structure.
Calibrated spectrophometry of the tail would be important for a determination of ion species
and densities, their variations, and their correlation with solar-wind events.
Finally, although I have stressed ground-based observations, there is absolutely no substitute
for in situ measurements. All we can do here is keep our fingers crossed.
To sum up, the plasma phenomena in comets are exciting, varied, and very worthy of future
study. Magnetic reconnection (Niedner and Brandt 1978), the flute instability (Ip and Mendis"
1978), the kink mode (Hyder, Brandt, and Roosen 1974), and the Kelvin-Helmholtz instability
(Ershkovich 1979), are but a few of the exciting plasma processes proposed to operate in comets.
Future observational programs devoted to plasma studies in comets have the chance not only to
increase our understanding of astrophysical plasmas, but of the structure of comets themselves. ,
References
Combi, M. R., and Delsemme, A. H. 1980, Ap. _J. 238, 381.
Delsemme, A. H., and Combi, M. R. 1979, Ap. J_. 228, 330.
Ershkovich, A. I. 1979, Planet. Space Sci. 27_, 1239.
Hyder, C. L., Brandt, J. C., and Roosen, R. G. 1974, Icarus 23, 601.
Ip, W-H., and Mendis, D. A. 1978, Ap. ±. 223, 671.
Niedner, M. B., Jr. 1980, Ap. jJ. 241, 820.
v
Niedner, M. B., Jr., and Brandt, J. C. 1978, Ap_. J>. 223, 655.
. 1979, Ap. _J. 234, 723.
. 1980, Icarus 42, 257.
32


Observational data needs for plasma phenomena

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server