Oxygen Injection Events observed by Freja

M. Yamauchi

IRF-Kiruna

* Motivation / Examples

* Distribution (mainly in nightside)

* Unusual events (dayside)

Phobos-2 (ASPERA) observation of two type of oxygen escape

We know that oxygen is escaping with much higher rate than thermal escape model predicts

==>

However, oxygen budget between Earth and space has barely been studied despite its obvious importance on:

* evolution of Earth, planets, satellites

* modelling ancient Earth (astrobiology issue)

* environmental issue

* energy and mass transfer from the solar wind

* plasma and molecular circulation, etc.

Why does Earth have so much Oxygen while Mars have so little?

Oxygen Budget/Circulation:

O+ supply from oxides (e.g., CO2 and SiO2).

* Oxygen escape / energization mechanism:Thermal (Jean) escape vs Non-thermal escape (night & cusp)

* Role of the escaped oxygen in space:e.g., Mass-loading effect by escaped ions.

* Oxygen circulation in the inner magnetosphere:If oxygen does not return, Earth should have lost its atmosphere.Simplest assumption = geomagnetic field push it back

* Oxygen precipitation into the ionosphere:Very few works ==> this is today's topic

Commonly-found ion injections near or equatorward of the nightside auroral zone.

(a) With heavy ions dominant.

Commonly-found ion injections near or equatorward of the nightside auroral zone.

(b) With proton dominant.

Analyses is simple and straight forward

Look at all the data and * Check the distribution and statitistics* Further study on the exceptional cases

But no work was published during past 15 years except 1 cusp paper.

I don’t understand why.

Distribution of heavy ion injections in 1° x 1h bin. Total number of traversals with clear injection events at each bin is scaled by the size of the square. The number with # is the orbit number.

Distribution of heavy ion injection events at 0.1-10 keV range.

* Nightside preference

* Some exceptions in dayside

This is not the artifact of Freja orbit with inclination of 63°because most injections are found inside CPS, a region far equatorward from the polar cap boundary.

Unusual event #1 and #2: Kp=7+, IMF ≈ 50 nT.

AE and ASY indices (12-hour plot) before the event #1 and #2:

The thick vertical line indicates the Freja observation of the injection event.

#1. Midday Heavy Ion Injection Event

* during extremely high magnetospheric activity * strong dawnward IMF => cusp (≈ substantial magnetic noon) is shifted toward dawn

* Mono-energetic heavy ion injection (0.3-1 keV range) is observed at 13 MLT (13:31:30 UT). * Eenergy band (width in %) of injecting ions is about the same for proton and heavy ions ==> only one species despite during mgnetic storm.

* The energy ratio between the injecting heavy ion and proton species is constant at about 15-20. ==> O+ rather than N+ or C+ if originate from the same region.

* This is not the stagnant plasma injection (SPI) event which is recognized at 13:28:25-13:29:20 UT in the proton channel.

Two possible interpretations for monotonic energy-time dispersion

(a) TOF case :

∆T = HTOF / V//2 - HTOF / V//1

or

HTOF = ∆T * V//2 * V//1 / ∆V//

note : pitch-angle dependence

(b) simple VF case

L1,2 / VC = HVF / V//1,2 , L2 - L1 = ∆T * Vsat

therefore

HVF / V//2 - HVF / V//1 = ∆T * Vsat / VC or

HVF = HTOF * Vsat / VC

L1,2 = HTOF * Vsat / V//1,2

Downgoing ions of ≈ 300 eV (60 km/s for O+) at L=7 :0.32 keV@13:31:30 UT & 0.18 keV@ 13:32:06 UT (6 spin)

==>

* Time-of-flight (TOF) distance ≈ 1 ReUnrealistic

* Velocity filter (VF) distance > 10 Re Location: ==> equitorial/opposite hemisphere

Peculiarity

* Dayside location==> Where do they come from?

* Heavy ions are registered first (at higher latitude) before protons are registered (at lower latitude).

==> What is the mass-filtering mechanism?

* Only O+ but not N+ despite storm condition==> What causes N+ escape at all?

* Only this event during entire Freja mission==> What is the peculiarity of this period?

Possible sources and transport routes for the dayside heavy ions:

Discussion on event #1

(1) Peculiarity of this period?Unusually strong dawnward IMF (≈ 50nT)Major storm (but many other periods as well)

(2) Source location * nightside ==> difficult* dayside same hemishpere ==> difficult* opposite hemosphere = cusp! ==> possible

(note: northern/southern cusp = prenoon/postnoon)

(3) Source mechanism?* Non.thermal escape from the cusp (but why selective to O+?)

(4) Mass-filtering mechansim? ????

#2. Heavy Ion Injection Adjacent to the Cusp

* during extremely high magnetospheric activity

* Another heavy ion injection is observed adjacent to the cusp (13:27:20 UT). * Among more than 100 cusps observed by Freja, this is the only one with heavy ion injection at different energy from H+* the oxygen energy is lower than the proton energy (both at 13:27:30 UT) contrary to the common cases observed by FAST (McFadden et al., 2003).==> We cannot identify the source mechanism and the source region for this specific case:

* drifted ions from the nightside?* escaping ionospheric ions reflected at high-altitude cusp?* escaping ions from the opposite hemisphere?* ???

Freja (h=1700km) - FAST (h=2000-4000km) discrepancy: Only FAST detected O+ during magnetic storms ==> possible explanations are:

* Downgoing O+ in the cusp is higher than the upper energy threshold of Freja (e.g., ring current origin)?

* Loss process (e.g., mirror reflection and charge-exchange) right above Freja is more important for O+ than for H+?

* Thermalization right above Freja is so strong that Freja can register H+/O+ in a wide energy range to overlap each other?

* Strong O+ injection occurs only for special magnetic storms.

At present we have no answer to the FAST-Freja discrepancy.

==> To solve this problem, we need another low-altitude satellite (< 2000 km) with a 3-D mass-resolving ion spectrometer with high energy and pitch-angle resolutions.

Unusual event #3: Kp = 6-.The third panel is energetic (10-100 keV) ions

#3. Multiple Heavy Ion Dispersion Event

* Two outstanding features in the heavy ion channel: (1) wedge-like dispersed structure of trapped oxygen at

around 18:47 UT (52-58 CGLat) (2) the overlapping and diverging energy-time dispersions

at 18:50:10 - 18:52:30 UT (61-65 CGLat).

* They are injections (downgoing count >> upgoing count) * No H+ signature = exclusively a heavy ion (O+ or N+) event.* They are multiple injections with different dispersion strength.

18:50:10 UT (0.56 keV => 0.13 keV in 12s) => TOF=10s18:50:25 UT (1.0 keV => 0.24 keV in 24s) => TOF=25s18:50:40 UT (2.4 keV => 0.32 keV in 75s) => TOF=45s

==> Simultaneous start times (18:49:55-18:50:00 UT)* TOF distance < 1000 km* No similar observation during entire Freja mission.

Same event: blow-up of 3 different sectors’ heavy ion data:

±45° pitch-angle, upward direction, and downward direction.

AE and ASY indices (12-hour plot) before the event #3 :

The thick vertical line indicates the Freja observation of the injection event.

* Rather in the oblique direction than in the downgoing direction* Detected during one of two half-spin periods (non-gyrotropic). note: O+ gyro frequency at 1700km ~ hundred Hz

==> Phase-mixing when travelling > 1s ==> Yet not gyrotropic==> Are they really injection events or heating by wave?==> But interpreting then as wave signature is even harder.

Non-gyrotropic distribution + downgoing flux suggests very anisotropic source (beam-like) and: (1) perpendicular convection is extremely large (~ 100 km/s),

==> very unlikely(no signature of fast convection)(2) Anisotropy of the source distribution was maintained ~ 1min.

==> single source with three directions of beams==> Injection must has large perpendicular component.

(3) Very close source (several tens gyro period <1s, i.e., <100km)==> multiple source

Short source distance (<1000km) makes it difficult to be detected by other spacecrafts (e.g. Viking and FAST) , and the ground.

Victoria (closest station from the foot-point of Freja during event ) does not show any specific intensification.

Summary of heavy ion injection events

* Surveyed entire Freja data (1992.10-1994.10) * Nightside preference* Some unusual event in dayside (1) O+ injection at 13 MLT followed by H+ injections.

VF source > 10 Re (TOF distance ~ 1 Re). From the southern dayside ionosphereMass-filtering mechanism is unknown.

(2) O+ injection (lower energy than H+) adjacent to the cuspFreja-FAST discrepancy

(3) Overlapping heavy ion injection with diverging dispersion In oblique non-gyrotropic directions

Sudden localized anisotropic energization at < 1000 km.Generation mechanism is completely unknown.