Morphology of Inner Magnetospheric

low-energy ions

M. Yamauchi, et al.

Swedish Institute of Space Physics (IRF), Kiruna

Ion drift under strong B-field (e.g. inner magnetosphere)

Magnetic drift (Energy dependent)

* gradient-B drift

* curvature drift

⇒ dominant for > 10 keV

ExB drift (Energy independent)

* co-roration E-field

* external E-field

⇒ dominant for < 0.1 keV

Ion drift

Energy dependent for magnetic drift and Energy independent for ExB drift/corotation

external E-field

Ion drift

Westward drift for high energy/evening and Eastward drift for low-energy/morning

Ion drift

Variation in the source and E-field makes the ion population a zoo of various patterns

Characteristic of Cluster Perigee

* North-South symmetric

* Quick scan of all latitude

The examined ion signatures (1)

(b) Wedge-like energy-latitude dispersed ions at sub-keV range: predominantly found in the morning sector some hours after a substorm. The source population is much colder than the plasma sheet. (both symmetric/asymmetric)

(e) Internal asymmetry of (b)

(c) Vertical stripes: similar to the wedge-like structure but dispersion is weak (nearly vertical in the spectrogram). The energy extends from the about 10 keV to sub-keV. (both symmetric/asymmetric)

The examined ion signatures (2)

The examined ion signatures (3)

(d) Short bursts of low-energy ions isolated from the above structures: a peak energy flux less than 100 eV but not thermal. (both symmetric/asymmetric)

The examined ion signatures (4)

(f) Warm trapped ions confined near equator: ~ tens eV to a few hundred eV.

(b) Wedge-like energy-latitude dispersed ions at sub-keV range: predominantly found in the morning sector some hours after a substorm. The source population is much colder than the plasma sheet.

(c) Vertical stripes: similar to the wedge-like structure but dispersion is weak (nearly vertical in the spectrogram). The energy extends from the about 10 keV to sub-keV.

(d) Short bursts of low-energy ions isolated from the above structures: a peak energy flux less than 100 eV but not thermal.

(e) Internal asymmetry of (b)

(f) Warm trapped ions confined near equator: tens eV to a few hundred eV.

The examined ion signatures (1)-(4)

Characteristic of Cluster Perigee

* North-South symmetric

* Quick scan of all latitude

+north-south symmetric nature of trapped ions (due to bouncing) Distribution must be north-south symmetric

Where? Inner Magnetosphere at 4~6 RE (Cluster perigee)

Species? H+ of 10 eV ~ 10 keV (CIS/CODIF energy range)

Distribution? Intense ion population (except plasma sheet)

In this work: (a) Statistics distribution, (b) 1-2 hour scale evolution/decay (using inbound-outbound asymmetry), (c) relation to substorms (using elapsed time from high AE),(d) modeling (using Ebihara’s simulation code).

We examined all SC-4 perigee pass during 2001-2006 (about 670 traversals, with relatively clean data of 460 traversals).

Analyses

Before analyses, we have to examine possible “biases” such as Instrument Degradation or Solar Cycle phase

Database quality check

We examine both “inbound-outbound” symmetric & asymmetric cases

Decrease of “wedge-like” is not due to solar cycle !

Occurrence of substorm is rather constant

The same for “low-energy burst”

Occurrence of substorm is rather constant

But “vertical stripes” show some solar cycle effect

Rather, the decrease is due to instrumental effect

Rapid degradation of the sensor

Occurrence of substorm is rather constant

Impossible to take statistics during 2003-2004 because of radiation dose. Otherwise large decrease.

How about warmed trapped ions?

Rapid degradation of the sensor

Occurrence of substorm is rather constant

(1) Both 2001-2003 & 2004-2006 show morning peak We can safely use 2001-2006 data together.(2) Very high observation frequency in the morning (> 80%)

Next check:Is Local Time distribution affected by this bias?

Local Time distributionsy

mm

etric

+ a

sym

met

ric

enha

nced

with

in tr

aver

salWedge-like:

high-occurrence (morning) peak is not midnight enhance > symmetric > decay

Vertical stripe: midnight phenomena enhance (midnight) decay ~ enhance (other LT)

Low-energy burst: all local time enhance ~ decay time scale ~ 1 hour

enhance/decay factor 3 change

Is enhancement related to substorm?

AL

AU

AL

AU

eastward drift

Viking 14 MLT

poleward

6 MLT

9 MLT

12 MLT

15 MLT

18 MLT

For the wedge-like dispersion, the relation to substorm has already been studied:

After AE>400 nT activity,

(1) Moves eastward

(2) Decay in time

hr from substorm

cf. Viking

Rel

atio

n to

AE

act

iviti

es

Loc

al T

ime

dist

ribut

ion

* Strong preference during |AL|>300 nT activity* Outstanding at midnight Related to substorm-related injction * Optimum at 300 nT threshold (cf. 200, 400 nT)

Vertical stripes

Rel

atio

n to

AE

act

iviti

es

Loc

al T

ime

dist

ribut

ion

* Same result as Viking (morning~0h, afternoon>5h)* Less enhancement with elapsed time stagnation* Yet, enhancements after 5h & aftrnoon ???

Wedge-like dispersion

Rel

atio

n to

AE

act

iviti

es

Loc

al T

ime

dist

ribut

ion

* slight decrease of “change” with time* but stagnation

Low-energy burst

The inner magnetospheric low-energy ion populations (1)-(3) below show significant changes within 1-2 hours.

(1) Wedge-like energy-latitude dispersed ions (< a few keV),

(2) Vertical stripes (< 10 keV),

(3) Short-lived low-energy ion burst (< few hundred eV).

• For all three patterns, asymmetric cases (large inbound-outbound difference) are found more often than symmetric cases at almost all LT.

• For vertical stripes, majority of local midnight pass show large inbound-outbound difference when AL<300 nT.

• For low-energy burst, the lifetime is estimated as short as 1 hour

Summary 1

Where? Inner Magnetosphere at 4~6 RE (Cluster perigee)

Species? H+ of 10 eV ~ 10 keV (CIS/CODIF energy range)

Distribution? Intense ion population (except plasma sheet)

In this work: (a) Statistics distribution, (b) 1-2 hour scale evolution/decay (using inbound-outbound asymmetry), (c) relation to substorms (using elapsed time from high AE),(d) modeling (using Ebihara’s simulation code).

We examined all SC-4 perigee pass during 2001-2006 (about 670 traversals, with relatively clean data of 460 traversals).

Next

Rel

atio

n to

AE

act

iviti

es

Loc

al T

ime

dist

ribut

ion

* Same result as Viking (morning~0h, afternoon>5h)* Less enhancement with elapsed time stagnation* Yet, enhancements after 5h & aftrnoon ???

Wedge-like dispersion

Why inbound-outbound difference of “wedge” at Noon-Afternoon sector so often?

⇒ We examine using numerical simulation (Ebihara’s code)

The ion drift alone can re-produce significant inbound-outbound difference !

0.1 keV

t ≈ 15 hr

Another example: sub-keV (dispersion asymmetry) and a few keV (ion band)

0.1 keV

t ≈ 6 hr

t ≈ 4 hr

The significant changes of the “ Wedge-like dispersed sub-keV ions within only 1-2 hours can be explained by the drift motion even in the noon-afternoon sector.

But some signatures are yet naturally explained by the local inflow from ionosphere, that also has short time scale.

For both sources, the ion energy is very low (< 0.1 keV) but not cold.

Summary 2

Future work: * Mass dependence* Pitch-angle dependence

note: // >> does not necessarily mean ⊥ionospheric source but mirroring ions

Next

equator

Confined near equator = Only direction

spin axis

12

4

5

78S

C m

otion

sectors ≈ PA

E(H+) << E(He+) : new

equator

equator

equator

All Local time

All AE conditions

Energy-time dispersion = new

Appearing in 40 min = new!

statistics & dynamics

Summary 3We also studied the “equatorially-trapped warm ions” which are basically symmetric between inbound and outbound

• The energy of He+ is often higher than that for H+.

• Some events show a non-thermal ring distribution rather than superthermal pancake distribution.

• Some events show energy-time dispersion, indicating drifts from different local times.

• The time scale of the development is again about 1 hour.

• At about 4-4.5 RE, the observation probability is about 1/3 (night-dawn) to 1/2 (noon-dusk).

• The He+/H+ ratio is variable and is much less than 5% for the majority of the cases.

Future work:

(2) (n+(1/2))fce wave is accompanied

* relation to wave

Future work: * relation to wave* compare different SC